I tried to google and check forum for the answer but I couldnt find anything like this

I have quite fresh installation of Release 7.2.0
I noticed number of deferred emails as it is a spam sent to unknown recipient from fake accounts. All seems to be normal and I just ignore it but recently I received warnings about viruses in attachments what clearly tell me that email first is processed by AV then MTA check if user exist.

It doesn't make any sense to me and is an additional task for my server what I would like strongly to avoid.
How I can change this process and denied all email to unknown users in first step and do not process it anymore.