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This one had me stumped for quite some time and is clearly a bug in SM v16 Enterprise:
- Symptom: All external emails sent to new email accounts would result in the Incoming Gateway rejecting them with "No Such User Here". Previously created email accounts are able to receive external emails just fine. Internal emails are not adversely affected and can be delivered successfully to all accounts, old and new.
- Configuration: This is using an Incoming Gateway with Domain Forwarding with manually defined "Specified Domains" (as the "All (Web Service)" setting is still broken in the current version of v16 causing the Incoming Gateway to always respond with a "No Such User Here" for all accounts and my Support Ticket to Smartertools for this issue being closed without a fix or resolution other than "don't use that setting").
- Cause: Turns out the cause of the error even when using "Specified Domains" was that in the MANAGE > FEATURES the CATCH-ALL ALIAS setting for that domain was enabled. If no Catch-All address is assigned by the Domain Administrator then all newly created email addresses are rejected by the Incoming Gateway with "No Such User Here" although all previously created email addresses are still able to receive email just fine.
- Resolution: Turning off CATCH-ALL ALIAS setting immediately resolved the issue and external email was able to be delivered to newly created email addresses again.
Definitely sounds like an unintended feature to me. Hopefully this will help anyone else stumped with why they are getting this error on valid accounts.