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Mail Services

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  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: 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 25 • Opens a TCP connection from the sending machine to the MTA listening on port 25 on host

  16. Sample communications S: 220 ESMTP Postfix C: HELO S: 250 Hello C: MAIL FROM:<> S: 250 Ok C: RCPT TO:<> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: Subject: test message C: From: C: To: 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" for the HELO command line: • S: ESMTP {postfix version and date} • S: 220 NO UCE. {etc., terms of service} • C: EHLO • S: Hello [] • S: 250-SIZE 14680064 • S: 250-PIPELINING • S: 250 HELP • Here, 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: 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 <> 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