Using backports.org is very simple:
1. Add this line
deb http://www.backports.org/debian sarge-backports main contrib non-free
to your /etc/apt/sources.list.
2. All backports are deactivated by default. You have to set a higher priority for each backport. For mutt, you add these lines:
Pin: release a=sarge-backports
to your /etc/apt/preferences (if you haven’t use pinning before, the file has to be created).
3. Then run apt-get update and apt-get install mutt as usual.
Another way is to skip the entry in /etc/apt/preferences and to use apt-get -t sarge-backports install mutt which is easier when the package requires additional backports.