The last stable release of SquirrelMail was July 12, 2011 according to wikipedia, which was nearly 14 years ago. I hope you're not still using it or considering using it.
For a small/medium SME setup postfix (SMTP) and dovecot (IMAP, POP) work together very well, for Web UI/UX you can integrate RoundCube.
PS - Must do - enable the server's NTP client to get time from a reliable NTP server. Dovecot used to stop w/o warning when it detected time drift.