What is SquirrelMail?
SquirrelMail is stable enough to use in a production system. It is, in fact, already used in several production systems around the world, handling thousands of users per system. There might be some bugs - no project is perfect - but they are most likely minimal. We test pretty thoroughly before releasing a version marked as "stable". More about Scalability: http://www.squirrelmail.org/about/scalability.php