Single account is unable to send mails to our Windows/Exchange 2003 domain.

Recently we discovered that a single customer was unable to send emails to anyone in our domain. However, she was able to successfully receive mails from all users in our domain  Also, all other users in her domain did not have a problem sending mails to our domain, it was just this single account.

The error message she received back was: #5.0.0 SMTP; 550 Requested action not taken: mailbox unavailable> #SMTP#.

As mentioned, she receives this error when sending mails to any user in our domain.

We are currently running MS Exchange 2003 on a Windows 2003 domain.

Does anyone have any idea why only a single user would be unable to send mails to our domain? Her individual user account is not blocked on our Exchange server or by the firewall.

I am not fully proficient on the server side, being desktop support,  but have been asked to look into the matter by my manager. I do though have access to the servers to check solutions.

Any help would be greatly appreciated.
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JSSYSConnect With a Mentor Commented:
This has happened to me when someone was trying to send an email to a specific person at another company.  Some how her email address was blocked at the other company but she could receive email from them.  We received the same message that you received.  The work around I used was to change her default email reply address when I did that the emails went through.  So you need to see if they can change the default reply address at the other company.  I am not sure what the root cause is but it must be spam filter related.
She is sending the email to an account that most likely does not exist (typo).
Try sending her an email and have her reply by only pressing reply and send.
First try this: Close Outlook, click Start->Run, type Outlook /cleanprofile
If that doesn't do the trick then recreate the Outlook profile. Profile corruption is the source of many strange behaviours in Outlook.
You may first want to save away the address cache (the .nk2 file) which contains the addresses the user has prevoiusly sent successfully. You find the .nk2 file here (XP):
C:\Documents and Settings\<user>\Application Data\Microsoft\Outlook
(Vista: C:\Users\{username}\AppData\Roaming\Microsoft\Outlook)
If you do not know how to recreate a profile:
After recreating the profile you can restore the .nk2 file to the same location assuming you used the same name for the new Outlook profile.
cscb08Author Commented:
Thanks for replys.

Kyosh: The email address she is sending to is definitely correct as I have the faxed sheet in front of me containing the error and she also forwarded the address to her IT department who in turn were successful in sending us mail.

peakpeak: Her profile is also uncorrupted as she can still send messages to every other person in her address book. It is only our domain where she is encountering the problem.

JSSYS: Your problem looks like the exact same one I am encountering and I also believed it was something to do with spam filtering. I will ask them to try this and post back.

Thanks again all.
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