When comparing Squirrelmail vs PrivMX WebMail, the Slant community recommends PrivMX WebMail for most people. In the question“What are the best self-hosted webmail clients?” PrivMX WebMail is ranked 6th while Squirrelmail is ranked 8th. The most important reason people chose PrivMX WebMail is:
Easy install and did not conflict with postfix dovecot roundcube system on FreeBSD.
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Pros
Pro Very mature
Squirrelmail has been around for a long time.
Pro Has many available plugins
Can do things like change Linux user passwords, configure vacation "out of office" messages, configure procmail .forward files, etc.
Pro Very easy to install
Easy install and did not conflict with postfix dovecot roundcube system on FreeBSD.
Cons
Con Very old
Squirrelmail has been around for a loooooong time!
Con Can't send regular email
This appears to be completely standalone system, only users of PrivMX can send and receive mail from other users with PrivMX. You can't send email to regular @ email addresses, or presumably receive email from them either.