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Old 3 Dec 2019, 11:56 AM   #6
Intergalactic Postmaster
Join Date: May 2004
Location: Irving, Texas
Posts: 8,706
Arrow Troubleshooting SMTP 550 No Such User Here error

Since we are already discussing your issue here in this general forum, I would not double-post. Maybe a moderator could move this thread to the Fastmail sub-forum.

Originally Posted by rmmarsh View Post
... I'm using as my email provider (Postbox as the email client)... also, should I move this to the Fastmal sub-forum? Added: here's the message I do get tho': 550 No Such User Here"
550 is a SMTP error code normally issued by a SMTP MTA (message transfer agent) server.
Error 550 is usually returned when the recipient "TO" mailbox can not be reached, for example if the TO address was incorrect or did not exist at the given domain on the receiving email system. But it can also be generated if your email client can't properly submit the message due to some problem (typically authentication of your email client due to an incorrect username or password or server port or SSL/TLS encryption problem).

So to troubleshoot this:
  1. First remove your Sieve rule to stop any confusion it may cause.
  2. Then try sending from that email client to your main Fastmail email address (not an alias).
    • If that fails, then there is probably something wrong with your email client configuration. Do not continue with the steps below.
    • If that works properly with no errors, then your problem must be due to something related to the TO email address, and you can continue troubleshooting.
  3. Next send from that email client in exactly the same way that worked in (2), but change the TO address to the one you have been testing.
    • If this works, then I don't see why you are getting the 550 message. The Sieve commands should not cause this message to be generated.
  4. If this fails, then carefully examine the TO address.
    • Is it your primary Fastmail address?
    • Is it an alias address which is properly enabled to target your Fastmail account?
    • Is it an alias address which instead targets an external non-Fastmail address?
    • Is it a subdomain address which is improperly formed?
My guess is that the message isn't getting properly submitted by your email client for some reason.

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