I have further reviewed this case and the emails are being held due to Suspicious Message Structure, this could be due to reasons like Mail formatting that should not be sent over the internet, Incorrect encoding of messages, this indicates that the formatting of the message does not meet RFC standards.

The sending environment is causing the issue with the message structure, message boundaries are out of place – this is something the sender will have to fix on their side. Unfortunately, there is no fix for this but can mitigate with a message passthrough policy.

To configure the message Passthrough policy:

1. Go to Administrator > Gateway > Policies.

2. Click on Message Passthrough Policy.

3. Click on the New Policy button.

4. Give the Policy a Narrative that describes its purpose.

5. On the Select Option, select “Do not explode Message Content”.

6. On the Emails From section, set:

Addresses Based On: The Return Address

Applies From: Individual Email Address

Specifically: Specify the email address

7. On the Emails To section, set:

Applies To: Internal addresses