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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!

  • Perform packet captures of the SMTP traffic at various locations, particularly between your anti-spam firewall and your Exchange server. My bet is it will be clear what's happening after examining those logs. – EEAA May 10 '16 at 4:14
  • Thanks, what should I use for packet capture, WireShark? – SamAndrew81 May 10 '16 at 13:07
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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. enter image description here

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