Microsoft: Multiple types of invalid user bounces

Sending mail to Microsoft domains can mean you get different types of bounces back for what is effectively the same problem. Check out these three different bounces, as an example:

  1. Your message to couldn't be delivered.
    firstname.lastname wasn't found at
  2. 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
  3. 550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302). [ 2023-09-19T15:05:34.324Z 08DBB71928CA8F8E]
  4. <>: host[] said: 550 5.4.1 Recipient address rejected: Access denied. [ 2023-09-18T12:48:32.808Z 08DBB45899852BA7] (in reply to RCPT TO command)

All of these are essentially "user unknown" or "invalid user" bounces, returned from attempting to send mail to an invalid or closed address at a Microsoft-hosted email domain.

The first one is the more "plain english" version that Microsoft builds into the DSN (Delivery Status Notification) that O365 will send back in some cases.

The second one? Seems self explanatory. O365 again, very clearly saying "recipient not found."

The third one? This is your typical "user unknown" bounce when sending to an invalid address at Microsoft OLC (Outlook Consumer -- aka Hotmail aka

The last one seems to generate the most confusion. Either because of its specific wording, or perhaps because it's a bit rarer, in my experience. It still means user unknown, but basically, the Microsoft server tried to lookup the destination address in a company's directory, and failed to find a match. 99.9% of the time, that means "invalid address," though I have heard (but not personally witnessed) that this can perhaps occasionally happen intermittently, as a false positive, if the directory system Microsoft is trying to use to find the address is not responding.

And now you know!



  1. Is "550 5.5.0 Requested action not taken: mailbox unavailable" definitively a bounce? Or can it be Microsoft saying that in lieu of blocking? Thanks.

    1. When I see it happening, it's definitely a "no such user" bounce. Could it also be used in some sort of spam blocking scenario? It's possible, but I doubt it.


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