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!
Found the issue, it was on our Untangle UTM. Max message size was far too low (1 MB), plus TLS session were not being allowed.