Building a deb package from source is relatively easy (note you want the DEB source not RPM) but you need to be careful with library dependencies. The new mutt might need library versions not available for the old Debian installation.
Inspect package requirements from the package page at https://tracker.debian.org/pkg/mutt . Notice the versioned links. The "box" symbol links the control file, which lists Build-Dependencies.
The most recent version 2.2.4-1 requires
Build-Depends: debhelper-compat (= 12),
Notice some of these have version numbers in the package name. Are they available for Debian 7? If not, it's going to be a nightmare iterating recursively down the chain of Build-Dependencies, probably not worth it. The next oldest version packaged for Debian was 2.0.5, not recent enough for your requirements.
If Debian 7 does happen to have the required libraries (libgnutls28-dev, libgpgme11-dev, etc), then the next step would be to download the orig tar file (the upstream source tarball), the debian .dsc file and the debian diff file (.debian.tar.xz), all available from the package page at https://packages.debian.org/unstable/mutt.
If you download those 3 files (and maybe the orig.tar.gz.asc for checksum), then you would unpack with
dpkg-source -x mutt_2.2.4-1.dsc
dpkg-source is in the
Then cd into the source dir (mutt-2.2.4). If all build dependencies are in order, all you would need to do is run
The .deb binary will be created in the parent directory, which you can install manually,
sudo dpkg -i mutt_2.2.4*.deb
You'll likely have problems with the required library versions listed in Build-Dependencies. I expect that it will not work on Debian 7 (without rebuilding the entire chain of libaries, which I'd judge as not worth the effort).
Apart from that you'd probably have a problem with
Build-Depends: debhelper-compat (= 12), which is more easily dealt with. Replace
debhelper-compat (= 12) with
debhelper (>= 9~) and create a file
debian/compat containing simply the number
For good measure you'd want to create your own debian/changelog entry to mark your local build with its own version number, which you can do using
debchange from the