An email sent outbound by one of your users is bounced back by Symantec.Cloud advising that the message could not be delivered.
This is the mail delivery agent at messagelabs.com.
I was unable to deliver your message to the following addresses:
This bounceback indicates that we accepted the message but we could not deliver it to the intended recipient server. We will include the refusal message that the recipient server provided which is the best source of information regarding the cause of the rejection. Some examples of issues and corresponding error messages:
552 5.3.4 Message size exceeds fixed maximum message size
552 Message size exceeds fixed limit
552 Mailbox is full
552 Requested mail action aborted: exceeded storage allocation
550 User unknown
554 5.7.1 Recipient address rejected: No such user
550 5.7.1 Unable to Relay
554 5.7.1 Relay access denied
550 5.7.1 Message rejected as spam by Content Filtering.
554 Denied (Mode: normal)
550 Denied by policy
552 MS-Office file containing VBA macros found inside of the email
550 5.7.1 Message rejected due to content restrictions
554 5.1.0 Sender denied
553 Envelope sender rejected
550 SPF check failed. Sender not authorized
550 Sender verify failed
550 5.7.1 SPF unauthorized mail is prohibited.
554 5.4.7 [internal] exceeded max time without delivery
Please note that this list is not exhaustive and the error messages from a server may not match exactly to the above, though they are often very similar.
The majority of these issues must be resolved on the recipient system as it is that system that is refusing the messages. It is also possible in some cases that you might need to confirm the recipient's email address or adjust the message content to prevent it from being blocked by the recipient server.
If you are unsure if the bounce is due to Symantec Email Security.Cloud blocking the mail, you can use Track and Trace to search for the particular email and review scanning service and delivery information to confirm this information.
In the case of SPF/Sender verification issues, you might need to implement or adjust your SPF Record. Please see: https://support.symantec.com/en_US/article.TECH226211.html
If you are unable to identify the cause for the message failure, please open a case with Symantec Technical Support and we can assist in understanding the issue with troubleshooting and message logs and advise what might be needed to resolve the issue.