FastMail has a great feature called personalities. This is especially useful if your FastMail account is receiving email for multiple domains. Unfortunately, Apple's iPhone and iPodTouch don't make this capability easy to take advantage of.
Fortunately, this is easy to fix. The latest iPhone firmware (version 3.x) makes this even simpler due to the inclusion of the copy and paste capabilities.
Apple's Mail.app program on the Mac has always supported this feature.
In Mail.App, you simply separate all of your email addresses with a
comma and they act similarly to multiple personalities.
The problem with the iPhone/iPod Touch? The keyboard is modified so that you cannot enter a comma in the email address field. Very frustrating. There are two solutions...
To fix the iPhone directly.
When you compose a new email on the iPhone, just click on the "From field" and choose the appropriate address.
To learn more how to configure the iPhone to work with FastMail, read this article.