Overview of Internet Mail

---

The major extensions to e-mail enabled it to contain more of the world's alphabets, encapsulate attachments, and permit multimedia files to be transmitted. Collectively, these are known as the MIME standards and are described in RFCs 2045-2049. Keep in mind that MIME encodes the arbitrary data as a sequence of ordinary characters using schemes such as Base64.

The SMTP design, which harkens from a simpler era, makes each message a self-contained morsel, but it's also the reason why spammers can be so successful. As the proposed update to SMTP explains in RFC 2821:

   Various protocol extensions and configuration options that provide
   authentication at the transport level (e.g., from an SMTP client to
   an SMTP server) improve somewhat on the traditional situation
   described above.  However, unless they are accompanied by careful
   handoffs of responsibility in a carefully-designed trust environment,
   they remain inherently weaker than end-to-end mechanisms which use
   digitally signed messages rather than depending on the integrity of
   the transport system.

Furthermore the MIME standards provide the basic mechanism whereby arbitrary binary data can be transmitted and malicious authors have been able to deceive some mail clients into decoding and executing programs which has made e-mail the main transmission vector for viruses.

However, the bottom line is that messages are simple flat character data and most of the mail system can be manipulated with ordinary editors and tools.

• MTA - Mail Transport Agent
  Moves the mail around the internet communicating via SMTP over reserved TCP/IP port 25. Since the mail protocols are all defined in ASCII, it is still perfectly possible to do "telnet someplace 25" and type in SMTP commands directly. Traditional MTAs were set up to cheerfully accepted mail for anyone, but spammers learned that they could use the MTA as an open relay to forward their messages. Now virtually no MTA will accept a message from outside its domain unless either (a) the message is to be delivered inside the domain or (b) the connected session has done a login to the MTA. Since login usually requires a password, such sessions may be encrypted using SSL technology. The original, and still very widely used MTA, is sendmail. It is a monolithic program combining an MTA, MDA, and even the outbound part of an MUA. Sendmail has a notoriously arcane configuration file. Other popular MTAs include qmail, postfix, and exim.
• MDA - Mail Delivery Agent
  Delivers mail locally according to the instructions of the user or system defaults. Programs like procmail and fetchmail are popular because of their ability to apply filters to the mail. Most MTAs actually include a default MDA to put mail in a single incoming folder known as a mailspool and/or the MTA will consult a user file which configures other options to forward messages to another host or program.
• MAA - Mail Access Agent
  As computing has become more decentralized and mobile a new kind of agent has appeared to provide remote access to the mailspool and mail folders. These servers look like MUAs to the system they're running on, but they are really accepting instructions from remote programs.
  • POP(port 110) is the original MAA and it simply delivers all messages from the incoming mailspool to the remote MUA. The remote system has the option of deleting the messages from the spool or leaving them. Since only incoming messages need be retained, POP is the service offerred by most ISPs.
  • IMAP(port 143) works with a remote MUA accepting commands to manage folders, download messages, move them to folders, or delete them.
    
  Both POP(port 995) and IMAP(port 993) accept encyrpted connections.
• MUA - Mail User Agent
  Sends mail to the MTA and retrieves messages from the mailspool. It also manages the mail folders on behalf of the user. This the most visible part of the system and the one that interacts with humans. There are too many MUAs to list. Here are examples from the major categories:
  • traditional text
    mail, mailx, elm, pine, mutt.
  • windowed GUI
    Netscape Messenger, KMail, Eudora, Microsoft Outlook family.
  • webmail
    IMP, SilkyMail, SquirrelMail,TWIG
  Many of the traditional text programs can only handle local mail whereas the GUI clients also speak POP and IMAP. Since the webmail programs run transiently inside browsers, they usually are configured to talk to IMAP servers.

• mailbox - a flat file with many messages
  There are a half-dozen or so variants in this category, but the general idea to just append individual messages to the end of a file. However, the simplicity of this format makes integrity complex when adding, or even worse, deleting messages. Typically the MUA or MAA has to lock the entire file during manipulation. This process is fraught with danger when done over NFS file systems which have a long and checkered history in file locking. There are allegations that folders with lots of messages consume a lot of resources as the MUA and MAA constantly reread and re-sort the contents so the system doesn't scale. For a list of the format variants and an absolutely convincing argument that mailbox is better than maildir, see the UW IMAP documentation page Mailbox Format Characteristics.
• maildir - directories with one message per file
  There are a few variants of this, most notably the qmail version, but all of them share the characteristic of one file per message. Reliability of mail handling is accomplished without file locking because this format leverages the automatic serializations built into every file systems for creating, renaming, and deleting files. There are allegations that the overall system serializations invoked during changes to file system metadata mean that the system won't scale. For an absolutely convincing argument that maildir is better than mailbox see the Qmail page Using maildir format.
• db - relational databases
  These are typically found in commercial-grade mail systems and make a lot of sense for managing myriad accounts and lots of simultaneous active users. One can leverage database technology for searches and transactions. However databases require a fair amount of care and attention, and are not transparently accessible by editors. For an absolutely convincing argument that database mail is the best, talk to a salesman of a commercial e-mail product.