identities don't work / can't send mail

[Killerhamster]

Hi. First of all - I just love this project, roundcube is so organised and stylish.

I installed it yesterday, I'm using a aol email Adress, because it's the only one I know, that's free and supports IMAP, plus it has 2 gigs.
The AOL adress works perfect for me, I can receive and send emails, but I was using a Gmail account before, so I just set Gmail on forwarding all messages to my AOL account. That works too, but I want to keep my Gmail Adress, so I need to be able to send mails from that account, which is supported by roundcube, but when I want to send a mail, and choose the Gmail Account, it won't send, it just gives me an error message.

Did I forgot to set something up in the configs, or why is it not working for me?