The error occures during mail sending....ofcourse

This appears to affect iCal (3 and 4) as well. If an iCal user mistakenly misspells the user part of an email address on the same domain as themselves. iCal then responds with:
'The server responded with
“HTTP/1.1 502 Proxy Error”
to operation CalDAVScheduleEventQueueableOperation.'
[ Go Offline ] [ Revert to Server ] [ Go Online ]

iCal can continue to be used if the 'Revert to Server' button is selected. However, after that, the event involving that attendee can't easily be deleted, because iCal's only option for deleting an event is 'Delete and Notify'. When the user clicks 'Delete and Notify', SOGo attempts to send an email to the misspelled email address and that sogod process dies with an SMTPException.
To reproduce this on the demo server:
1) Setup iCal 3 or 4 to connect to it with CalDAV.
2) Create a new event.
3) Attempt to invite: afsdfsf@sogo-demo.inverse.ca
4) Click Send.

This issue still exists in recent nightly builds. I'm running into this in 1.2.2.20100707 if a user enters an incorrect address on the local domain.

We'll fix the issue eventually, do not worry.

In the meantime, if you want to avoid the issue, you could set the mailing mechanism to "sendmail" instead of "smtp".

This is fixed. The webmail no longer display a livid page and the exception in question is no longer raised.