Every so often I need to be able to check IMAP account settings. This can be done by manually connecting to the IMAP server with telnet.

Once connected, all IMAP commands are preceded with a word of your own choosing, which the server will respond with. This is to enable the client to recognise the reply to each command which it has sent (this would be useful where a real mail client sends various commands to the server without waiting for a reply after each one, and can later identify the reply to each command).

Some useful commands are:

LOGIN [username] [password]
LIST [flags] [folder separator] [search term]
STATUS [mailbox] [flags]
SELECT [mailbox]
FETCH [first]:[last] flags
FETCH [mail number] body[header]
FETCH [mail number] body[text]
LOGOUT

Here is an example of an IMAP conversation between telnet and the server:

telnet: > open imapserver.example.com imap
telnet: Trying 192.0.2.2...
telnet: Connected to imapserver.example.com.
telnet: Escape character is '^]'.
server: * OK Dovecot ready.
client: a1 LOGIN MyUsername MyPassword
server: a1 OK Logged in.
client: a2 LIST "" "*"
server: * LIST (\HasNoChildren) "." "INBOX"
server: a2 OK List completed.
client: a3 EXAMINE INBOX
server: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
server: * OK [PERMANENTFLAGS ()] Read-only mailbox.
server: * 1 EXISTS
server: * 1 RECENT
server: * OK [UNSEEN 1] First unseen.
server: * OK [UIDVALIDITY 1257842737] UIDs valid
server: * OK [UIDNEXT 2] Predicted next UID
server: a3 OK [READ-ONLY] Select completed.
client: a4 FETCH 1 BODY[]
server: * 1 FETCH (BODY[] {405}
server: Return-Path: sender@example.com
server: Received: from client.example.com ([192.0.2.1])
server:         by mx1.example.com with ESMTP
server:         id <20040120203404.CCCC18555.mx1.example.com@client.example.com>
server:         for <recipient@example.com>; Tue, 20 Jan 2004 22:34:24 +0200
server: From: sender@example.com
server: Subject: Test message
server: To: recipient@example.com
server: Message-Id: <20040120203404.CCCC18555.mx1.example.com@client.example.com>
server: 
server: This is a test message.
server: )
server: a4 OK Fetch completed.
client: a5 LOGOUT
server: * BYE Logging out
server: a5 OK Logout completed.

This post gives you more details on syntax and examples of more of the IMAP commands:
http://wiki.mediatemple.net/w/Email_via_IMAP_using_Telnet