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Microsoft Windows OS (XP/NT/Vista/Windows 7/8/9/10) Forum

550 "Unknown User"

 3:01 pm on Feb 5, 2010 (gmt 0)

I'm using outlook 2007.

If I mail to a certain e-mail address and I click on "send", I instantly receive an email saying that the mail send didn't reach the recipients.

With the following error:

'editor@example.com' op 5-2-2010 15:47
550 "Unknown User"

It only seems to happen on my outlook address, if I use my hotmail or gmail (both webmail) then everything works fine.

The editor@example.com can mail me, that works fine. If its used to mail me on my outlook address and I reply the same thing happens, I get the 550 error.

If I check the email properties, the headers box is completely empty. It seems that the email doesn't even reach my SMTP server.

I can't remember where, but I read somewhere that you can "empty" the outlook cache to solve problems like this, now I searched Google and I figured out how to clear the "Download Cache temp folder" and the "Email Address Auto Complete Cache" but those aren't things I'm looking for.



 7:34 am on Feb 6, 2010 (gmt 0)

Could the AutoComplete address or Contact address you're using contain the wrong e-mail address?


 1:49 pm on Feb 6, 2010 (gmt 0)

Building on Bill's suggestion...

Email addresses are often written as
"Display Name"<username@example.com>

Perhaps you have something like


Robert Charlton

 3:06 am on Feb 8, 2010 (gmt 0)

Perhaps you have something like

You don't have to have <junk> in the address area to have a problem with this. In my experience, the "@" character in the display name area is enough to get an email immediately kicked back, as in...

UserName@example.com <username@example.com>


 7:16 am on Feb 8, 2010 (gmt 0)

Even when I reply to an email? And one of my co-workers has the same problem.


 7:54 am on Feb 8, 2010 (gmt 0)

What we're suggesting is that you may have the wrong address stored in your cache or address book. It may look correct, but may be concealing an incorrect address. If you are using Exchange or another shared address book then it certainly would be possible for your colleagues to have the same problem.

Robert Charlton

 8:01 am on Feb 8, 2010 (gmt 0)

Even when I reply to an email?

Often especially when you reply to an email. I've run into a number of situations where emails would get kicked back because the return addresses had been set up as I described above....

UserName@example.com <username@example.com>

That "@" character is an "illegal" character in user names, but apparently it doesn't affect senders... only those who reply to it.


 8:06 am on Feb 8, 2010 (gmt 0)

Hmm ok.

I removed the address form the auto complete list. Then I typed it in again, sending a new email (not a reply) and the same thing happends. I'm 100% I typed in the correct address.

We don't use a shared address book.


 1:35 pm on Feb 8, 2010 (gmt 0)

100% sure that is*


 12:01 am on Feb 9, 2010 (gmt 0)

You need to experiment...

Try username1@example.com and username@example1.com, etc. and see what happens.



 7:08 am on Feb 9, 2010 (gmt 0)

Not sure I'm getting you kaled, but I used your example:

username1@example.com and username@example1.com ofcourse with the email address that's not working not with example1.com :P

And the username1@example.com fails with the "550 "Unknown User"" error and the username@example1.com fails with a "Mail delivery failed: returning message to sender".

Not sure what this means though...


 10:46 am on Feb 9, 2010 (gmt 0)

In the case of the "Unknown User" error, did you check the header as you did previously? If it's blank again that would be interesting.



 10:10 am on Feb 10, 2010 (gmt 0)

The username@example.com header is empty
The username1@example.com header is empty
The username@example.com1 header isn't empty, it tells me where it goes wrong and why it is returning the message to me (which is correct) and I get a proper mail delivery failed message as well.


 10:16 am on Feb 10, 2010 (gmt 0)

This happened to me last night. It was text in the body of the email that caused it. I removed the text and had no issues...apparently a particular word was the culprit.


 10:18 am on Feb 10, 2010 (gmt 0)

Opps, sorry the message I was getting was: 554 Message refused...totally mis-read this thread.


 12:53 pm on Feb 10, 2010 (gmt 0)

The username1@example.com header is empty

So that rules out caching issues.

This suggests that the problem is somehow related to the domain name - are there any non-ascii characters in it?
What happens if you place the domain name in uppercase?
What happens if you place the username in uppercase?



 1:09 pm on Feb 10, 2010 (gmt 0)

Thank you for the help kaled!

There are no weird chars in the domain name, only a to z chars.

What happens if you place the domain name in uppercase?
What happens if you place the username in uppercase?
They both instantly return with a 550 "Unknown User" error.


 1:46 pm on Feb 10, 2010 (gmt 0)

What happens if you omit the user name, i.e send to simply @example.com

Does this domain name have a catchall redirection set up, i.e. if you send to non-existent-name@example.com should it be redirected elsewhere?



 7:28 am on Feb 11, 2010 (gmt 0)

Outlook won't let me do that.

If I mail to info@domain.com, which exists, everything works fine. It seems to work fine anyway...


 2:16 pm on Feb 11, 2010 (gmt 0)

Outlook won't let me do that.
It's a domain control panel feature, not an Outlook feature.

The reason I asked was I was wondering if Outlook might be clearing the to: header in response to an error.

I presume, since you are getting an immediate error, that you are using an smtp server on the domain name in question. If you use the smtp server provided by your ISP instead, I suspect the problem will go away.



 8:27 am on Feb 12, 2010 (gmt 0)

the smtp server I use is not from my provider nor is it the domain I'm trying to mail to.


 11:33 am on Feb 12, 2010 (gmt 0)

"Unknown User" is typically reported only for email boxes on the same domain as the smtp server. I believe it can be reported for remote domain names but only if both ends implement the necessary features.

I think changing the smtp server for the purpose of testing is still advisable.



 11:15 am on Feb 15, 2010 (gmt 0)

Thanks kaled,

I will give it a try. I've been rather busy with a host of other things lately. I will give an update of how it goes.


 11:31 am on Feb 15, 2010 (gmt 0)

I have a client with similar problems under Outlook.

Vanished when I installed Thunderbird and suggested he try that for a day or two.

Don't know why the problem exists, just know that there is at least one solution.

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