I suspect that the "sending address not accepted due to spam filter" is the biggest clue as to the problem. On your Smartermail Server go to SECURITY > ADVANCED SETTINGS > SMTP BLOCKING. It sounds like you have an address (or wildcard address) that is configured that is blocking that particular user from sending.
For example, we accidentally blocked a sender named firstname.lastname@example.org because we had a wildcard address of *cialis*@* to block male-enhancement spam.
Pro-tip The easiest way to find a SMTP Blocked wildcard address is to search (F7) for just a few letters in the first part of their email address.