webmail application for production?

[Anze Vidmar]

hello everybody!

Which webmail application would be most secure for the production
environment? 
I'm talking here from 300-5000 users all having there mailboxes on one
server, and they need a webmail-style access to their email.

I don't want to take any security risk here, so i really need a secure
solution. The security is a priority. Is squirrelmail secure enough?

I would like to hear your experiences on this one.

Thank you.

Anze