bizhub C3350, scan to mail doesn't work

I'm trying to make my bizhub C3350 scan directly to an email/contact, saved in the addressbook. Unfortunately this doesn't work at all. Everytime I try to send a document, the printer shows the notice: "An error occured. The job will be canceled" (free translation from the german error message).

I double checked the settings in the Web Connection:

- Within the System Preferences I set the administrator email
- Network/Email-Settings should be right. I tested three different mail accounts, that work fine when testing the settings on my Mac. I even checked the settings with my KM dealer via Teamview. He couldn't find any faults. We even testet one of his maill settings, that work fine on the printers in his shop.
- The manual isn't helpful either.

Propably the problem is a missing setting on my router (FritzBox 7490): All the ports, needed for the mail delivery delivery are opened for the printer. Maybe there is another setting I have to change?

I know, it's hard to help on network stuff, but maybe someone of you has a flash of genius.

Thanks a lot!
Philip