brew install mutt

It appears the mutt ftp server is down, but a minor change to the brew formula made things work:

$ rcsdiff /usr/local/Library/Formula/mutt.rb 
===================================================================
RCS file: /usr/local/Library/Formula/RCS/mutt.rb,v
retrieving revision 1.1
diff -r1.1 /usr/local/Library/Formula/mutt.rb
5c5
<   url 'ftp://ftp.mutt.org/mutt/devel/mutt-1.5.21.tar.gz'
---
>   url 'http://prdownloads.sourceforge.net/mutt/mutt-1.5.21.tar.gz?download'