It's a common story : you have to find the reason for the most unexpected problem with your mail server , but it's not a common story : you have to find the reason for receiving more than 10 emails per minute because of sending virus which has been happend after installing DrWeb!
The problem began when we installed DrWeb and activated it for just only one mailbox (e.g. email@example.com) . And then we received thousands of emails with the same subject "Undelivered mail" and the messages clarified that there is a big problem with our account(!) because it was sending virus to firstname.lastname@example.org account.
We assumed that it's a right claim and we need to look after it. There was a question : if our account sent virus to email@example.com and now after installing anti-virus, DrWeb could detect it , why firstname.lastname@example.org didn't receive any virus when we disabled DrWeb for account?
At the end we found out that there is no virus sending and it's just because of DrWeb malfunction :
" There seemed to be a looping issue caused from a mailbox delivery event that may have been configured within the Plesk control panel. The delivery event was executing when messages that were received in the mailbox email@example.com which created a notification message directly into the SMTP outbound queue which had the same recipient as the mailbox that sent it. This caused the message to be delivered and fire the event again, causing a loop. We disabled the delivery event to stop this. The developers are currently communicating with Plesk to resolve the problem, as it seems that the notification messages being created by the delivery event do not have "Precedence: Bulk" in the headers which will stop the loop (since the delivery event will not then be fired as it shouldn't for system generated messages). "
This problem with DrWeb will be solved by using this patch provided by SW-Soft