Daemons, testing via Telnet

http://www.textolonotaald.com The telnet client can be used to interact directly with service daemons which use human-readable communications protocols - such as IMAP, SMTP, POP3, or HTTP - and make sure they're doing what they are supposed to do. This is an extremely valuable troubleshooting tool, as the experienced admin can see the exact communication from the server rather than rely on "friendly" but often inaccurate error messages given by client utilities intended for end users.

For more information, see the articles for testing each service:

SMTP, testing via Telnet

IMAP, testing via Telnet

POP3, testing via Telnet

HTTP, testing via Telnet