Gaetan
"We don't accept email with executable content [UEsDBAoAA]"
Well that is a ZIPv1.0 format, & if you allow that you will receive lots & lots of virus infected messages, which are sent as ZIPv1.0 format.
Virus infected messages/attachments are so prevalent in ZIPv1.0 format.
My suggestion would be to use RAR format to compress files, ie WinRAR, as there appears to be a lot less nasty messages/attachments in RAR format.
If your users compress a ZIP file into a RAR format, then the ZIPv1.0 (or whatever) will still be detected by SME server, so you need to start with a non ZIP format source.
Alternatives are to put files on a web site for sharing, & just email the link, or create an external free email account/address, & get external senders to send email to that address when attachments are involved eg files in formats that SME will block. This email address should not be publicly advertised or you will just get lots of spam etc coming to it. You could also create a webshare or similar upload/download site for sharing these types of files, particularly good for really large files eg in the 50Mb to 200Mb range where some users have data files of that size.
Really security comes first, you cannot risk allowing your server & network to get infected, as the downtime cost is too great to tolerate for most businesses (both unproductive staff time costs as well as tech support costs), but you need to cater for ease of use by customers who will send large attachments & attachments in formats that SME server will block, alternatives for this already suggested. I have this issue in my business & a combination of the workarounds mentioned seems effective enough.
Unfortunately Stefanos attitude of just don't do it is not so practical. Businesses need to make it as easy as possible for clients to liaise with us, but my suggestions have been practical to most clients & they catch on quick when told what to do.