E-Mail, newsgroups, and forums

Formats for the storage or transmission of messages (singly or in groups) for electronic mail, discussion lists, forums, newsgroups, BBSs, social networking, etc. See also Web, Address books and contacts.

Abbreviations, conventions, etc. in messaging
Online messages can be a language all their own, with different dialects for different types of messages, but often "leaking" from one service to another. Here are a few of the things to watch out for:
 * Emoticons - smileys, frowneys, etc :-)
 * Hashtags, at-signs, retweets, etc. - Twitterisms that often turn up elsewhere
 * Txtspeak - various abbreviations sometimes used in chats

Bulletin Board Systems (BBSs)

 * Fido BBS
 * FidoNet message packet
 * FidoNet nodelist
 * Opus-CBCS (BBS software)
 * Opus Embedded Commands (OEC)

Chat, Texting and Instant Messaging

 * AOL Instant Messenger
 * ICQ
 * IRC (Internet Relay Chat)
 * SMS (text messaging on phones)
 * XMPP (Jabber)

E-Mail

 * AOL
 * AOL e-mail messages
 * AOL Personal Filing Cabinet (includes e-mail as well as other things)
 * Blue Wave
 * Eudora
 * Internet e-mail message format (RFC 733/RFC 822/RFC 2822/RFC 5322)
 * Maildir
 * mbox
 * MLM (Novell Groupwise)
 * Pegasus Mail
 * POP
 * PST - Outlook Personal Storage Table
 * QWK
 * SMTP
 * SOUP

Forums

 * Discourse
 * phpBB

Secure messaging

 * Crypho
 * DeadDrop
 * Silent Circle Instant Messaging Protocol

Social Networking

 * Facebook
 * Friendster
 * Google+
 * Instagram
 * Myspace
 * Pinterest
 * pump.io
 * Tumblr
 * Twitter

Transfer Encodings

 * Base64
 * MIME
 * Mime-type
 * Quoted-printable
 * Transport Neutral Encapsulation Format (TNEF; winmail.dat)
 * UUEncoding
 * yEnc

Usenet

 * NZB
 * Usenet message format (RFC 850/RFC 1036/RFC 5536/RFC 5537)

Links

 * Online, Underground: In-progress written history of online messageboards