mutt and fastmail.fm

It is possible to open IMAP folders in mutt without doing any further configuration. To test this, run mutt in a terminal window and press c to open a mailbox. Enter your Fastmail.fm mailbox in this format: 

imaps://mail.messagingengine.com/

When prompted, enter your FastMail username and password. While this is good for initial testing, it would be quite cumbersome to do this every time you wanted to change mailboxes. The solution is to place your IMAP server settings in the mutt configuration file $HOME/.muttrc. If you don't have this file, create it. Add the following entries: 

set imap_user=<your fastmail user>
set imap_pass=<your fastmail password>
set record="imaps://mail.messagingengine.com/INBOX/Sent Items"
set postponed=imaps://mail.messagingengine.com/INBOX/Drafts
set imap_home_namespace="INBOX"
set folder="imaps://mail.messagingengine.com/INBOX"
set spoolfile="imaps://mail.messagingengine.com/INBOX"
set mask="."

mutt will now go directly to your FastMail account on startup and won't ever ask for a username or password. This will also save a copy of all your sent and postponed messages on the server. This matches the settings in Mail.app (assuming you followed my instructions above) and in the FastMail web interface. This means that you can start editing a message in FastMail, save a draft, and then open it in Mail.app or mutt with no problems. 

It's Working--Now What?

You are now set up to receive mail using all three applications. However, you probably can't send mail using mutt yet. The reason is that mutt, unlike many other mail programs, does not come with an integrated facility to send mail. Instead, it relies on your system mailer (sendmail). That works fine on traditional Unix systems. However, desktop computers are usually not configured with sendmail to send mail directly. 

You have two choices here. The first is to configure a full mail transfer agent (MTA) such as sendmail or postfix. This is usually overkill for a desktop system because it also sets up your system to receive mail, which you probably don't need (or want). A lighter-weight alternative is a send-only sendmail replacement such as sSMTP or esmtp. I recommend esmtp because it supports SMTP authentication. You will need authentication with most SMTP servers (FastMail requires it). esmtp is currently not available via Fink, so you will need to build it from source. You can download the source from the esmtp sourceforge site. After you install the esmtp program, create the configuration file $HOME/.esmtprc containing the following (assuming you will be using the FastMail SMTP server): 

hostname = mail.messagingengine.com
username = <your fastmail username>
password = <your fastmail password>

And add the following to your .muttrc: 

set sendmail=/usr/local/bin/esmtp 

Now mutt will hand outgoing messages off to esmtp. esmtp will then authenticate with the FastMail SMTP server and relay the message.