When comparing Atmail vs Squirrelmail, the Slant community recommends Atmail for most people. In the question“What are the best self-hosted webmail clients?” Atmail is ranked 4th while Squirrelmail is ranked 5th.
Ranked in these QuestionsQuestion Ranking
Pro Intuitive interface
Pro Powerful yet easy to use admin tools
Pro Browser, desktop and mobile versions
Pro Plugin support
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.
Con Bad UX
It's slow and unintuitive.
Con Closed source
Appears to be a proprietary product.
Con Not free
As in beer or software.
Con Very old
Squirrelmail has been around for a loooooong time!