Just found today when I was looking to see if I missed something with the mutt sidebar patch (still irritates me that I have to sync a mailbox before jumping to another mailbox in order for the counts in the sidebar to be updated properly), that mutt 1.5.15 was released earlier this month. This is an extremely worthwhile upgrade, especially because it now has SMTP support built-in. It's about time!
No more having to use exim or postfix to relay the mail, you can now send the mail out directly from mutt itself which makes things a whole lot nicer. The relevant section of the documentation covers it a little bit more, but I've just tested it out and it works great (you will want to compile it using --enable-smtp and --with-sasl). Works wonderfully with non-authenticated SMTP (good for using your ISP's mail server) and also works with authenticated SMTP (tried with my server which uses CRAM-MD5), and also supports using an email address as a login (i.e you could use "set smtp_url="smtp://firstname.lastname@example.org:email@example.com" and it works properly).
All in all, kudos to the mutt team for getting this in place! I'll have to update my wiki to cover this as it largely makes irrelevant the sections I have in there configuring postfix and exim to relay mail (which you can still do if you like).