I built a rpm to install squirrelmail. I have given this limited testing on 7.1.3 so please use AT YOUR OWN RISK on a TEST server only. Rpms will appear as soon as mirrors sync here:
http://mirror.contribs.org/smeserver/contribs/dmay/smeserver/7.x/testing/smeserver-squirrelmail/Most setting are already done for generic SME 7.1.3 compatibility. Follow normal squirrelmail documentation to configure/change any specific settings:
# cd /opt/squirrelmail/config
# ./conf.pl
In my testing this happily co-exists with Horde/IMP. YMMV.
Darrell