Why MQM colecting mails as spam to Unwanted content with reason "Mail format" (as Mail size filter in MEG)?
In Policy of MEG I set that Mails greater with 8 Mb will be just blocked (not copied in quarantine). But MQM collecting mail with an attachment least then 8Mb as Unwanted content.
If you have not done so yet, I would recommend to open a case with support on this. I have a feeling it is a configuration issue somewhere.