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 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...

  1. Create your account in Apple's Mail.App on your Mac with the comma
    separating the personality email addresses. Then resync your iPhone
    and choose to migrate the email account information.
  2. The second (and easier solution) is to take advantage of the
    cut/copy/paste capability in the iPhone 3.x firmware to paste into
    your settings.

To fix the iPhone directly.

  1. Open Notes, create a new note. Type in your other personality's
    email address (without quotes)... " , ". NOTE the
    comma, this is the most important part.
  2. Use the tap commands to select all and copy this information to the
  3. Open up the Settings/Mail, Contacts, Calendar application.
  4. In the IMAP Account Information Field, tap at the end of the Address
    field and Paste the additional email address. After you've done
    this, your Address field will look something like this:,
  5. You have to use this trick to get a comma into that field

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.