No Such User Here
Problem reported by Scarab - August 2, 2017 at 11:38 AM
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.

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There is one more thing with incoming gateways. Try to send any email to your gateway during main smtartermail instance is starting (system is up but mailservice is still starting). Incoming gateway will bounce sent message with no such user here. I've had this problem many times during some problems with system here main smartermail instance is running.
I've sent this as a support ticket and after some reviewing I've got response:
Our developers have reviewed this issue further and at this point it is going to be considered a feature request regarding the 550 no such users.
The issue is being caused when the backup MX loses it's list of user accounts on the primary server, this is stored in memory, so if the service is ever restarted when the primary is unavailable, this issue will be encountered. 
We will be adding a feature that writes this list to a flat file so that it can be queried against in the event the primary server is down. This should be included in SmarterMail 16, however I wasn't able to obtain a firm ETA. Please let me know if you have any questions or concerns.
IMHO problem is bigger for instances with higher accounts number which starts longer. After main server started I had to restart my 3 incoming gateways so they stop bouncing messages with no such user here error.
This behavior substantially defeats the usefulness of the gateway. The gateway should be retrying delivery using a set of pre-defined rules, just like any other SMTP delivery mechanism. And of course the list of addresses should be stored in a database, flat-file or otherwise.
We have also the "no such user" problem with the current Version 16.3.6621. Yesterday we updated our systems and today a bunch of customers complaining about. Our setup is similar to Scarabs. We also use incoming gateways.
It looks to me that this error only affects addresses of domain aliases. Addresses of "normal" domains don't seem to be affected. Also the error don't appearing all the time. The same user sometimes got a "no such user" and sometimes a "recipient ok". 

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