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)
If Google was an '80s BBS...

Programs

 * Fido BBS
 * Opus-CBCS (BBS software)
 * Opus Embedded Commands (OEC)
 * PCBoard

Networks, protocols and formats

 * AVATAR
 * EMSI/IEMSI
 * FidoNet message packet
 * FidoNet nodelist
 * FidoNet nodediff
 * FOSSIL
 * Wazoo
 * YooHoo

Chat, Texting and Instant Messaging

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

Standards and protocols

 * Habeas Warrant Mark
 * IMAP
 * Internet e-mail message format (RFC 733/RFC 822/RFC 2822/RFC 5322)
 * Maildir
 * mbox
 * POP3
 * QWK
 * SMTP
 * SOUP
 * SPF
 * T.37 (fax to e-mail)

Specific programs

 * AOL
 * AOL e-mail messages
 * AOL Personal Filing Cabinet (includes e-mail as well as other things)
 * Apple Mail
 * Blue Wave
 * Eudora
 * Gmail
 * Hotmail
 * Lotus Notes
 * Notes Storage Facility (NSF)
 * Mailpile
 * MLM (Novell Groupwise)
 * Outlook
 * Nickfile (NK2)
 * Personal Folder File (PAB, PST, OST)
 * Outlook Express Database (DBX)
 * Pegasus Mail
 * Pine
 * Thunderbird

Forums

 * Discourse
 * phpBB

Secure messaging

 * Crypho
 * DeadDrop
 * PGP
 * PKCS7
 * Privacy-Enhanced Mail (RFC 1421)
 * SafeSlinger
 * Silent Circle Instant Messaging Protocol
 * S/MIME

Social Networking

 * Facebook
 * Friendster
 * Google+
 * Instagram
 * LinkedIn
 * MySpace
 * Pinterest
 * pump.io
 * Snake
 * Tumblr
 * Twister
 * 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)

BBSing

 * Lots of vintage BBS software
 * Modems, wArEz, and ANSI art: Remembering BBS life at 2400bps
 * Elite Exchange One BBS software manual (1989)
 * BBS-related podcasts

Email

 * How to self-host your own mail server in the cloud
 * NSA Says It Can’t Search Its Own Emails (Just everybody else's!)
 * How can I find out where an email really came from?
 * New Encrypted Email Services Coming From Kim Dotcom's Mega And Newzbin
 * Dark Mail Alliance - developing end-to-end encryption for e-mail
 * TorBirdy: extension to use Tor encryption with Thunderbird
 * The Daunting Challenge of Secure E-Mail
 * Verify e-mail addresses online
 * Yahoo breaks every mailing list in the world including the IETF's
 * The End of History: How e-mail is wrecking our national archive (ironically, the article succumbed to linkrot, but is retrievable from the Internet Archive)

Newsgroups/Usenet

 * Kickstarting Operator: a censorship-resistant, shape-shifting newsreader

Social networking

 * LinkedIn ‘Intro’duces Insecurity
 * Free download of danah boyd's must-read book "It's Complicated" (about young people and social networking)

Misc.

 * Online, Underground: In-progress written history of online messageboards
 * Opt out of PRISM: surveillance-free alternatives to things including social networking and messaging
 * India to send world's last telegram. Stop.
 * PLATO: The Emergence of Online Community
 * The Minitel: France's 1980s "Internet"