Instead of doing pop3 why not set user2 as an alias to user1?
Then you can easily select which identity to send from and get all the mail in one location - use a filter if you want to sort the incomming

Admin console > individual user > aliases tab
Quote:
Originally Posted by leog21 I can't find how I can send mail out from user2 when I'm logged into user1. |
You'd have to enable 'allow sending from any address' then set up an identity, but setting up an alias automatically sets up/allows an identity to send from.
You could have forwarded as well, (and choosen not to keep the mail locally) but that's just like your pop setup.
Plus, on 4.5.x the web client does not automatically check pop on a schedule, you have to click the 'get mail' button (the RFE for automatic checking made it into 5.0).