We have the same problem since two days. It seems to be caused by SPam Mails...
--
In: RCPT TO: <
[email protected]>
Out: 550 5.1.1 <
[email protected]>: Recipient address rejected:
User unknown in virtual mailbox table
In: DATA
Out: 354 End data with <CR><LF>.<CR><LF>
Out: 451 4.3.0 Error...