Ok, so you've set up your own mail server on your PowerBook for when you're on the road, and don't know what ISP you'll be connecting to next. Now you can send mail from anywhere, as long as you've got an internet connection, and it'll go through just fine, right? Wrong. An incresing number of mail servers worldwide are restricting where they accept mail from, dynamic or dial up IP ranges are blocked in many cases, mail from IP addresses without a resolvable DNS name, or even ANY DNS name, etc. And some mail is just dropped, you'll never know it bounced. So, what to do, you ask?
The answer is to relay your mail through another mail server. Generally, this is something that most mail servers should be set up to avoid doing (as Postfix does), but in this case, what we want to do is get mail from our mobile mail server to a fixed mail server elsewhere. If your regular ISP has a mail server set up to allow SASL (Simple Authentication and Security Layer) access, Postfix can be configured to support this, and once authenticated, mail can then be relayed to your ISP's mail server, which should then go through fine (assuming your ISP has a properly configured mail server).
A second option would be if you've already set up Postfix as a mail server at home (as I have), and your ISP doesn't provide SASL access, would be to pass mail to your home system, which can then send the mail out. A bit convoluted, but it will get the job done, and you won't need to worry about mails not going through.
Not quite the simple plug and play solution Mac folks are used to, but mail servers are not known for being simple, nor should they be. Anyone attempting to set up their own server should understand the technology involved, and be prepared to support themselves, and probably consider joining one of the mailing lists available.