The best replacement for squirrelmail is horde.

The best alternatives to squirrelmail are rated as following:
★★★★★  horde
★★★★  roundcube
★★  openwebmail
★  zimbra
★  atmail
★  imp
★  sendmail
  neomail
  round cube

Your alternative for squirrelmail is not listed? Suggest!
Speaking about squirrelmail, one probably shouldn't forget about postfix.

Other search terms related to squirrelmail and their ratings are:
★★★★★ postfix
★★★ qmail
★★★ hmailserver
★★ dovecot
You have in mind a companion for squirrelmail and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of squirrelmail in all years. Click on graph for more.
squirrelmail
horde
roundcube