Have replicated this issue multiple times, from multiple email accounts. I have verified the email passes through the anti-spam firewall (Untangle), but Exchange Message Tracking doesn't even have record of the email coming in. Also, I do not see the test sender email address (my gmail) in our Exchange Logs (C:\Program Files\Microsoft\Exchange Server\TransportRoles\Logs\ProtocolLog\SmtpReceive), and we have "Verbose" logging already enabled on the Receive Connectors.
The only way I have been able to send in a PDF larger than 4 MB's is as follows: Linux VM in off-site test lab > SMTP2Go Relay Email service (with test lab IP whitelisted for anonymous authentication) > Our Exchange.
Any and all feedback on this appreciated. Thank you!