There are two main participants in email, servers and clients. Servers generally speak Simple Mail Transport Protocol (SMTP) to each other; to clients, they might speak SMTP, but they are more likely to communicate with IMAP or POP.
All of these protocols are somewhat similar, as you might imagine.
Connected to mail.cs.fsu.edu. Escape character is '^]'. 220 ***************************** helo mail.cs.fsu.edu 250 mail.cs.fsu.edu mail from:
250 Ok rcpt to: 250 Ok data 354 End data with . Subject: This is a test Test email. . 250 Ok: queued as 37A76F343C quit 221 Bye
openssl s_client -connect mail.cs.fsu.edu:995 [ .... lots of secure handshake information ... ] CONNECTED(00000003) +OK POP3 mail server ready user rlangley +OK User name accepted, password please pass ********* +OK Mailbox open, 10 messages list +OK Mailbox scan listing follows 1 1219 2 5033 3 4626 4 3767 5 3920 6 7581 7 8074 8 2105 9 8966 10 2243 . quit +OK Sayonara
openssl s_client -connect mail.cs.fsu.edu:993 CONNECTED(00000003) [ ... lots of secure handshake information ... ] * OK [CAPABILITY IMAP4REV1 LITERAL+ SASL-IR LOGIN-REFERRALS AUTH=PLAIN AUTH=LOGIN] mail IMAP4rev1 at Tue, 9 Apr 2013 15:45:16 -0400 (EDT) tag1 login rlangley ********* tag1 OK [CAPABILITY IMAP4REV1 LITERAL+ IDLE NAMESPACE MAILBOX-REFERRALS BINARY UNSELECT SCAN SORT THREAD=REFERENCES THREAD=ORDEREDSUBJECT MULTIAPPEND] User rlangley authenticated tag2 select INBOX * 10 EXISTS * 0 RECENT * OK [UIDVALIDITY 1351002924] UID validity status * OK [UIDNEXT 11] Predicted next UID * FLAGS (\Answered \Flagged \Deleted \Draft \Seen) * OK [PERMANENTFLAGS (\* \Answered \Flagged \Deleted \Draft \Seen)] Permanent flags * OK [UNSEEN 1] first unseen message in /var/spool/mail/rlangley tag2 OK [READ-WRITE] SELECT completed
In the Unix world, there are only a few text/text-like clients that are reasonably popular: