So, this one's a quick trick that I've known ever since my days of writing articles in Sys Admin, but it's a trick I hadn't thought about letting others know about for a while, until I was reminded by a recent post over at Hawk Wings.
Port 25 isn't the only port for sending mail on. In fact, there are two other common ports in use by most ISPs and mail servers:
Port 587 serves as the port for the submission protocol (RFC 2476) - this port is designed to take mail from end users, and functions exactly like SMTP for all intents and purposes.
Also, port 465 is the reserved port for SMTPS (SMTP over SSL) - this is an encrypted version of SMTP.
This is useful info because many ISPs (especially wireless or hotel ISPs) block outgoing traffic on port 25 to prevent spam. So, you can re-configure your mail client to send its outgoing mail on port 587 or port 465 (make sure the "secure connection" box is checked for 465), and you'll be able to send mail from almost any of your accounts, no matter where you are.