Mutt mudou seu comportamento em 1.5.21. Você pode encontrar os detalhes aqui:
http://marc.info/?l=mutt-dev&m=128478045511699&w=2
Mutt will now display most text/* parts internally via the pager. I'm guessing your system /etc/mailcap has a text/html entry without the "needsterminal" keyword that Mutt is using to render the text.
You can still get the old behavior in the attachment menu by pressing 'm' (view-mailcap) which will always use mailcap for viewing, and should pick up any entries for external viewers ("needsterminal").