1 / 61

Mail Services

Mail Services. Pre-survey: How many server/client programs (or services) do you need to send and receive email?. 0 1 2 3 more. Mail Services. Three major mail service protocols: Simple Mail Transfer Protocol SMTP Post Office Protocol POP3 Internet Mail Access Protocol IMAP or IMAP4.

boris
Download Presentation

Mail Services

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Mail Services

  2. Pre-survey: How many server/client programs (or services) do you need to send and receive email? • 0 • 1 • 2 • 3 • more

  3. Mail Services • Three major mail service protocols: • Simple Mail Transfer Protocol • SMTP • Post Office Protocol • POP3 • Internet Mail Access Protocol • IMAP or IMAP4

  4. Which one to use? • Outbound Mail • SMTP for outbound email • Port 25 or 2525 • Server/repository • Inbound Mail (clients) • POP3 for inbound email • Port 110 • IMAP for inbound email • Port 143

  5. What is SMTP? • Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending emails across the Internet • SMTP uses TCP port 25 or 2525 • Sometimes you may have problems sending messages • ISP may have closed port 25 • To determine the SMTP server for a given domain name • MX (Mail eXchange) DNS record is used

  6. IMAP or POP3? • Both receive email • Main differences between IMAP and POP3: • POP3 protocol assumes there is only one client (computer terminal) that ever connects to the mailbox • IMAP protocol allows simultaneous access by multiple clients (computer terminals) • Same user on the clients • IMAP is also suitable if the mailbox is used by multiple users

  7. Material from: http://en.wikipedia.org/wiki/Smtp SMTP

  8. SMTP • Simple Mail Transfer Protocol • The de facto standard for e-mail transmissions across the Internet • Defined in RFC 821 (STD 10) • amended by RFC 1123 (STD 3) chapter 5. • The protocol used today is also known as ESMTP • Defined in RFC 2821

  9. SMTP • Relatively simple text-based protocol • One or more recipients of a message are specified • Most cases verified to exist • The message text is transferred • Client-server protocol • The client transmits an email message to the server • Either • an end-user's email client, a.k.a. MUA (Mail User Agent), - or - • a relaying server's MTA (Mail Transfer Agents) • Can act as an SMTP client

  10. SMTP • An email client knows the outgoing mail SMTP server from its configuration • A relaying server typically determines which SMTP server to connect to by looking up the MX (Mail eXchange) DNS record for each recipient's domain name • The part of the email address to the right of the at (@) sign • Conformant MTAs (not all) fall back to a simple A record in the case of no MX • A Record: address record • Some current mail transfer agents will also use SRV records • SRV Records: SeRVice records • More general form of MX • These are not widely adopted • Relaying servers can also be configured to use a smart host

  11. SMTP • The SMTP client initiates a TCP connection • Server's port 25 • Unless overridden by configuration • Quite easy to test an SMTP server using telnet • see following example

  12. SMTP • SMTP is a "push" protocol • Does not allow one to "pull" messages from a remote server on demand • That is it sends only • To do a pull (i.e. receive) a mail client must use POP3 or IMAP • Another SMTP server can trigger a delivery in SMTP • Using ETRN

  13. Outgoing mail SMTP server • Email client requires the name or the IP address of an SMTP server as part of its configuration • Server will deliver messages on behalf of the user • Setting allows for various policies and network designs • End users connected to the Internet can use the services of an e-mail provider that is not necessarily the same as their connection provider • Network topology, or the location of a client within a network or outside of a network, is no longer a limiting factor for email submission or delivery • Modern SMTP servers typically use a client's credentials (authentication) rather than a client's location (IP address), to determine whether it is eligible to relay email

  14. Outgoing mail SMTP server • Can use either TCP port 25 (SMTP) or port 587 (Submission) for relaying outbound mail to a mail server • Established by RFC 2476 • Many servers support both • Some servers still support port 465 for legacy secure SMTP • Preferable to use encryption on standard ports according to RFC 2487 • Some servers are setup to reject all relaying on port 25 • Valid users authenticating on port 587 are allowed to relay mail to any valid address. • Server that relays all email for all destinations for all clients connecting to port 25 is known as an open relay • Generally considered a bad practice worthy of blacklisting

  15. Sample communications • After establishing a connection between the sender (the client) and the receiver (the server), the following page shows a legal SMTP session • In the following conversation • everything sent by the client is prefaced with C: • everything sent by the server is prefaced with S: • On most computer systems, a connection can be established using the telnet command on the client machine, for example: • telnet www.example.com 25 • Opens a TCP connection from the sending machine to the MTA listening on port 25 on host www.example.com

  16. Sample communications S: 220 www.example.com ESMTP Postfix C: HELO mydomain.com S: 250 Hello mydomain.com C: MAIL FROM:<sender@mydomain.com> S: 250 Ok C: RCPT TO:<friend@example.com> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: Subject: test message C: From: sender@mydomain.com C: To: friend@example.com C: C: Hello, C: This is a test. C: Goodbye. C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye

  17. Sample communications • Note: the data the client sends in the HELO and MAIL FROM commands can be retrieved in additional headers that the server adds to the message: • Received and Return-Path respectively. • Optional: (and not shown above) nearly all clients ask the server which SMTP extensions the server supports • Uses the EHLO greeting to invoke Extended SMTP (ESMTP) • These clients use HELO only if the server does not respond to EHLO

  18. Sample communications • Contemporary clients will use the ESMTP extension keyword SIZE to inquire of the server the maximum message size that will be accepted • Older clients and servers • Try to transfer huge messages that will be rejected • After wasting the network resources • Including a lot of connect time to dialup ISPs • paid by the minute

  19. Sample communications • For the sending of giant files or sending with older clients, users can manually determine in advance the maximum size accepted by ESMTP servers • The user telnets as above, but substitutes "EHLO mydomain.com" for the HELO command line: • S: 220-serverdomain.com ESMTP {postfix version and date} • S: 220 NO UCE. {etc., terms of service} • C: EHLO mydomain.com • S: 250-serverdomain.com Hello mydomain.com [127.0.0.1] • S: 250-SIZE 14680064 • S: 250-PIPELINING • S: 250 HELP • Here, serverdomain.com declares that it will accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). • Depending on the server's actual resource usage, it may be currently unable to accept a message this large

  20. Sample communications • In the simplest case, an ESMTP server will declare a maximum SIZE with only the EHLO user interaction. • If no number appears after the SIZE keyword, or if the current message limit must be exactly determined: • User can further interact by simulating the ESMTP header of a message with an estimated size. • See External Link RFC 1870

  21. Material From: http://en.wikipedia.org/wiki/Post_Office_Protocol POP

  22. POP • Local e-mail clients use the Post Office Protocol version 3 (POP3) • An application-layer Internet standard protocol • Retrieves e-mail from a remote server over a TCP/IP connection • Many subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3

  23. POP Overview • POP3 has made earlier versions of the protocol obsolete • POP (formally referred to POP1 and POP2) • Now, the term POP almost always means POP3 in the context of e-mail protocols

  24. POP Overview • Design of POP3 and its procedures supports end-users with intermittent connections • (e.g. dial-up connection) • Allows users to retrieve e-mail when connected • View and manipulate the retrieved messages without needing to stay connected • Although most clients have an option to leave mail on server, e-mail clients using POP3 generally: • Connect • Retrieve all messages • Store them on the user's PC as new messages • Delete them from the server • Disconnect

  25. POP Overview • Most e-mail clients support either POP3 or IMAP to retrieve messages • Few Internet Service Providers (ISPs) support IMAP • Fundamental differences between POP3 and IMAP4: • POP3 offers access to a mail drop • Mail starts on the server • Downloaded to client when accessed • Deleted from server • If the client leaves some or all messages on the server • The client's message store is considered authoritative • IMAP4 offers access to a mail store • Mail stays on the server after accessed • Client may store local copies of the messages • These are considered to be a temporary cache • The server's store is authoritative

  26. POP Overview • Clients with a leave mail on serveroption generally use POP3 UIDL (Unique IDentificationListing) command • Most POP3 commands identify specific messages by their ordinal number on the mail server • Creates a problem for a client intending to leave messages on the server • Message numbers may change from one connection to the server to another • For example if a mailbox contains five messages at last connect, and a different client then deletes message #3, the next connecting user will find the last two messages' numbers decremented by one • UIDL provides a mechanism to avoid these numbering issues • Server assigns a string of characters as a permanent and unique ID for the message • When a POP3-compatible e-mail client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers • The client can then use this mapping to determine which messages it has yet to download • Saves time when downloading • Note: IMAP has a similar mechanism, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending. • Advantage of the numeric UID is with large mailboxes • A client can request just the UIDs greater than its previously stored "highest UID" • In POP, the client must fetch the entire UIDL map

  27. POP Overview • Whether using POP3 or IMAP to retrieve messages: • E-mail clients typically use the SMTP_Submit profile of the SMTP protocol to send messages • E-mail clients are commonly categorized as either POP or IMAP clients, but in both cases the clients also use SMTP • There are extensions to POP3 that allow some clients to transmit outbound mail via POP3 • Known as "XTND XMIT" extensions. • The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client-to-server e-mail transmission. • MIME serves as the standard for attachments and non-ASCII text in e-mail • Although neither POP3 nor SMTP require MIME-formatted e-mail, essentially all Internet e-mail comes MIME-formatted • POP clients must also understand and use MIME • IMAP, by design, assumes MIME-formatted e-mail

  28. POP Overview • POP3 originally supported only an unencrypted login mechanism • Plain text transmission of passwords in POP3 still commonly occurs • POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail • APOP: Authenticated POP • Uses the MD5 hash function • Attempt to avoid replay attacks and disclosure of the shared secret • Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail and Novell Evolution • POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version

  29. POP Overview • POP3 works over a TCP/IP connection • Uses TCP on network port 110 • E-mail clients can encrypt POP3 traffic using TLS or SSL • A TLS or SSL connection is negotiated using the STLS command • Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995

  30. POP Example (APOP) S: <wait for connection on TCP port 110> C: <open connection> S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us> C: APOP mrose c4c9334bac560ecc979e58001b3e22fb S: +OK mrose'smaildrop has 2 messages (320 octets) C: STAT S: +OK 2 320 C: LIST S: +OK 2 messages (320 octets) S: 1 120 S: 2 200 S: . C: RETR 1 S: +OK 120 octets S: <the POP3 server sends message 1> S: . C: DELE 1 S: +OK message 1 deleted C: RETR 2 S: +OK 200 octets S: <the POP3 server sends message 2> S: . C: DELE 2 S: +OK message 2 deleted C: QUIT S: +OK dewey POP3 server signing off (maildrop empty) C: <close connection> S: <wait for next connection>

  31. POP Example • POP3 servers without the optional APOP command expect you to log in with the USER and PASS commands: C: USER mrose S: +OK User accepted C: PASS mrosepass S: +OK Pass accepted

  32. POP3 • Uses port 25 • Leaves email on the server by default • Allows the client to disconnect from the server, then work with the individual emails • Is more flexible than IMAP

  33. Resume 3/17

  34. IMAP

  35. IMAP • Internet Message Access Protocol • Commonly known as IMAP or IMAP4 • An application layer Internet protocol • Operates on port 143 • Allows a local client to access e-mail on a remote server • Previously called Internet Mail Access Protocol, Interactive Mail Access Protocol , and Interim Mail Access Protocol • Current version is IMAP version 4 revision 1 • IMAP4rev1: defined by RFC 3501 • IMAP4 and POP3 are the two most prevalent Internet standard protocols for e-mail retrieval • Virtually all modern e-mail clients and servers support both

  36. IMAP • IMAP supports both connected and disconnected modes of operation • E-mail clients using IMAP generally leave messages on the server until the user explicitly deletes them • Allows multiple clients to access the same mailbox • Clients in this case are different hosts • Most e-mail clients support both POP3 and IMAP • Few Internet Service Providers (ISPs) support IMAP • IMAP4 offers access to the mail store • Client may store local copies of the messages • Considered to be a temporary cache • Server's store is authoritative

  37. IMAP • E-mail messages are generally sent to an e-mail server • Stores received messages in the recipient's e-mail mailbox • User later retrieves these messages with either a web browser or an e-mail client that uses one of a number of e-mail retrieval protocols • Some clients and servers preferentially use vendor specific, typically proprietary protocols, most support the Internet standard protocols • SMTP for sending e-mail • POP3 and IMAP4 for retrieving e-mail • Allows interoperability with other servers and clients

  38. IMAP • Proprietary protocols: • Microsoft Outlook client when communicating with an Exchange server • IBM Notes client when communicating with a Domino server • Both products also support SMTP, POP3, and IMAP4. • Support for the Internet standard protocols • Allows other e-mail clients to access these servers • Allows the clients to be used with other servers • E.g. Qualcomm's Eudora or Mozilla Thunderbird

  39. IMAP • E-mail clients can generally be configured to use either POP3 or IMAP4 to retrieve e-mail and in both cases use SMTP for sending. • Most e-mail programs can also use Lightweight Directory Access Protocol (LDAP) for directory services • Many (if not most) subscribers to individual ISP e-mail accounts access their e-mail with client software that uses POP3 • IMAP is often used in large networks • For example, a college campus mail system

  40. IMAP • IMAP allows users to access new messages instantly on their computers • The mail is stored on the network • With POP3, users either • download the e-mail to their computer • access it via the web • Both methods take longer than IMAP • User must either download any new mail or "refresh" the page to see the new messages

  41. POP – IMAP Comparisons

  42. IMAP Advantages over POP3 • Connected and disconnected modes of operation • When using POP3, clients typically connect to the e-mail server briefly, only as long as it takes to download new messages. • When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand. • For users with many or large messages, this IMAP4 usage pattern can result in faster response times.

  43. IMAP Advantages over POP3 • Multiple clients simultaneously connected to the same mailbox • The POP3 protocol requires the currently connected client to be the only client connected to the mailbox. • The IMAP protocol specifically allows simultaneous access by multiple clients • Provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients

  44. IMAP Advantages over POP3 • Access to MIME message parts and partial fetch • Nearly all internet e-mail is transmitted in MIME format • Allows messages to have a tree structure where • leaf nodes are any of a variety of single part content types • non-leaf nodes are any of a variety of multipart types • The IMAP4 protocol allows clients to • Separately retrieve any of the individual MIME parts • Retrieve portions of either individual parts or the entire message • Mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched

  45. IMAP Advantages over POP3 • Message state information • flags can keep track of message state • For example: whether or not the message has been read, replied to, or deleted • Flags are stored on the server • Different clients accessing the same mailbox at different times can detect state changes made by other clients • POP3 provides no mechanism for clients to store such state information on the server • If a single user accesses a mailbox with two different POP3 clients state information cannot be synchronized between the clients • E.g. whether a message has been accessed • IMAP4 protocol supports both pre-defined system flags and client defined keywords • System flags indicate state information such as whether a message has been read • Keywords allow messages to be given one or more tags whose meaning is up to the client • (not supported by all IMAP servers) • Adding user created tags to messages is an operation supported by some Webmail services, such as Gmail • Generally not using IMAP

  46. IMAP Advantages over POP3 • Multiple mailboxes on the server • IMAP4 clients can create, rename, and/or delete mailboxes on the server, and move messages between mailboxes • Usually presented to the user as folders • Multiple mailbox support also allows servers to provide access to shared and public folders • Server-side searches • IMAP4 provides a mechanism for a client to ask the server to search for messages meeting a variety of criteria • Avoids requiring clients to download every message in the mailbox in order to perform searches • Built-in extension mechanism • IMAP4 defines an explicit mechanism by which it may be extended • Many extensions to the base protocol have been proposed and are in common use • IMAP2 did not have an extension mechanism • POP3 now has one defined by RFC 2449

  47. IMAP Disadvantages v. POP • IMAP remedies many of the shortcomings of POP •  inherently additional complexity • Much of this complexity is compensated for by server-side workarounds • e.g., multiple clients accessing the same mailbox at the same time • Maildir • Database backends

  48. Disadvantages of IMAP • If the mail store and searching algorithms on the server are not carefully implemented: • Client can potentially consume large amounts of server resources when searching massive mailboxes

  49. Disadvantages of IMAP • IMAP4 clients need to explicitly request new email message content • Potentially causing additional delays on slow connections such as those commonly used by mobile devices. • A private proposal, push IMAP, would extend IMAP to implement push e-mail by sending the entire message instead of just a notification • However, push IMAP has not been generally accepted • Current IETF work has addressed the problem in other ways

  50. Disadvantages of IMAP • Unlike some proprietary protocols which combine sending and retrieval operations, sending a message and saving a copy in a server-side folder with a base-level IMAP client requires transmitting the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder • Remedied by a set of extensions defined by the IETF LEMONADE Working Group for mobile devices • POP3 servers don't support server-side folders so clients have no choice but to store sent items on the client • Many IMAP clients can be configured to store sent mail in a client-side folder • Courier Mail Server offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder

More Related