The instructions and screenshots in this setup guide are for Mac OS X 10.9 (Mavericks). Previous versions of OS X are very similar, but may require more details to be entered manually. If you are using an older version of Mac OS X, you can find the information you might be asked for on our server names and ports page.
Open Mail.app. If you do not already have any accounts set up, it will prompt you to set up a new account. Otherwise, go to Mail → Preferences and select the Accounts tab. Click the + at the lower left of the screen. Select Add Other Mail Account…
Fill in your name, email address and password:
Click the Create button. The account will be created and Mail.app will start syncing your mail.
To use a different email address to your FastMail username:
In the Email Address field, replace your FastMail username with the address you wish to use instead. You can also enter multiple email addresses, separated by a comma. If you do this, Mail.app will let you choose which one you would like to use each time you compose a message.
If you change your password after adding your FastMail account to Mail.app, you will need to update the settings.
Click the Advanced tab, then focus the Password field, delete the current contents and type in your new password.
You should now be able to both send and receive mail again with your FastMail account. If you can do one, but not the other, try following the steps again carefully: your password may have been updated correctly for incoming mail, but not for outgoing mail, or vice versa.