1. Be greater than 32 bytes in size (the minimum size necessary to include all required parts)
2. Have a DATE: header
3. Have either a FROM: or a SENDER: header
4. If present, there can be only one FROM: header
5. If present, there can be only one SUBJECT: header

Messages can be exempt from compliance checks if:

1. They are from authenticated sessions
2. They are from a trusted IP
3. The FROM: is from a trusted domain 

Note: Customers using AntiSpam Maxim will find additional compliance checks which we will not disclose of obvious reasons. 

If you are still receiving a rejection message, and you've checked that your email as all the above requirements, then raise a ticket via the helpdesk and paste in the entire email source including headers, MIME boundaries and Encoding.