Messages receiving bounce back error for CRES when CRES is not being used

[anotthak8]

Messages receiving bounce back error for CRES when CRES is not being used. For example, a third party is sending a message with a zip file that is 20 MB. However, when the third party sends the message, the error  "Your message wasn't delivered to anyone because it's too large. The limit is 25 MB. Your message is 27 MB." and "SMTPSEND.OverAdvertisedSize; message size exceeds fixed maximum size" is received. Also, I viewed the message details and it stated that the message "Incoming connection (ICID 5036173) lost" for all messages from this user but the message was still delivered to the recipient.

It appears the third party is sending from the cloud using Office 365 and Exchange (exchangelabs.com). Cisco partners with Office365 to protect email and the Cisco envelope service has a 25MB limit. The cloud exchange server is telling them that we rejected it but CRES is not being used. Is it possible that since both sending and receiving server use the CRES product, that it is rejecting it even though CRES isn't being used?

Any insight or information will be helpful. Thank you!

[Mathew Huynh · Accepted solution]

Hello Anotthak,

Can i please ask if you could share some sample tracking or NDRs emails where this is seen.

This sounds like a message size limit being reached on the settings on the mail servers rather than CRES limits matched.

You can check your ESA's current message size limits at

GUI > Mail Policies > Mail Flow Policies and check the mail-flow policies which are matched.

The maximum message size here is the total message size of the email including the ~34% MIME inflation applied.

So for example,

ESA configured at 10MB limit here.

If an email is coming to your ESA that is 10MB including attachments.

The ESA will reject it as the ESA will add MIME inflation effectively making it ~13.4MB.

So to accept a full 10MB email on the ESA, your ESA mail flow size limit needs to be at least 14MB.

Regards,

Matthew