Michail Pappas
If you refer to this
http://wiki.contribs.org/Virus:Email_Attachment_Blockingand have a good careful read of it, you may be able to analyse a suitable signature for various file types within a zip file.
If possible or practical, then I would guess the signature would need to be quite long, much longer than the standard 9 or so characters, in order to cover a specific file type eg exe that is within a zip file.
Then add those signatures to the database, following the instructions in the wiki article.
Good luck and let us know if you are successful.
PS It's not hard to analyse the signatures, just follow the instructions. You just have to be careful to find a signature that is valid across all scenarios eg all exe files inside zip files and so on.
I should add, that the issue you describe is a good reason to block all zip file types in attachments, so that exe files are not accepted by the email system.
If you need to receive zip files of say large data files, use a seperate method to allow users to upload exe or zip files to your server, eg webshare and there are now others options available as contribs, or even an external POP mailbox that you ensure is virus scanned.