Mail Assure Troubleshooting
Why am I receiving NDR messages from Microsoft 365 for mailboxes which do not exist?
Wed Oct 07 13:48 GMT 2020
- Why am I receiving bounceback (NDR) messages, saying that delivery has failed, when the address in the NDR does not exist in Exchange online?
- These bounceback messages are showing that this is for an Email Scout Report (ESR) message.
- The error code (status reply) starts with: 550 5.1.10 RESOLVER.ADR.RecipientNotFound
- N-able Mail Assure
- Microsoft 365 - Exchange online
- This is due to Exchange Online only rejecting mail to invalid mailboxes, at the end of the DATA command. Not at the RCPT TO command.
- When Mail Assure is set to not reject mail to mailboxes not in the mailboxes tab, mailboxes are validated in real time against the receiving server.
- As Microsoft does not reject invalid mailboxes at the RCPT TO command, these technically do exist, so mail is accepted and filtered by Mail Assure.
- This will trigger any automated reports configured for the domain, causing the NDR when this is delivered.
- To prevent this behaviour:
- Ensure all valid Mailboxes and Mailbox Alias addresses are present in Mail Assure
- At domain level, under Mailboxes Configuration, Enable the option to Reject email to mailboxes not listed in the "Mailboxes" tab
- This will disable recipient callouts to the domain, causing the Mail Assure list of mailboxes to be authoritative.
- Any mail received to addresses not in this list (it is common for spammers to attempt random addresses) will be rejected by Mail Assure.
- Important: mail being accepted by the filter (due to a positive response to the RCPT TO command from the receiving MTA), will incur a filtered mailbox charge for each address which a message is received. To ensure that accurate billing is produced, ensure that the option to reject invalid addresses is enabled.