Messages deleted via POP3 aren't supposed to be moved to Trash. You're seeing a bug in the beta that will be fixed in the next release (coming very soon). Please check out this thread for details:
http://zimbra.com/forums/showthread....ighlight=trash
The minimum trash lifetime is currently hardcoded to 31 days. (The feature's not designed for what you were trying to use it for; it's run only once a night by default.) If you think this is too high, please let me know.