Reason: 554 5.4.7 [internal] exceeded max time without delivery

People,

Some of my users who regularly send out the email to the customer suddenly got this email NDR:

Reason: 554 5.4.7 [internal] exceeded max time without delivery

I wonder why MessageLabs block it?

Message Body

This is the mail delivery agent at Symantec Email Security.cloud.

I was unable to deliver your message to the following addresses:

Friend@apnoutdoor.com

Reason: 554 5.4.7 [internal] exceeded max time without delivery

The message subject was: Thank you
The message date was: Thu, 22 Feb 2018 04:54:20 +0000
The message identifier was: B0/5A-19486-20D4E8A5
The message reference was: server-12.tower-164.messagelabs.com!1519275264!53903131!1

Please do not reply to this email as it is sent from an unattended mailbox.
Contact your email administrator if you need more information, or
instructions for resolving this issue.