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Recently we've upgraded a clients website from our IIS6 server to our new IIS7 server and they are receiving the following error on occassion when sending out emails:

Mailbox unavailable. The server response was <> No such user here

The client has confirmed that the email address they are sending to is valid and exists.

What other reasons could there be for this error?

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up vote 1 down vote accepted

However insistent the client might be that the address exists, you can't argue with a message from the mail server responsible that says it doesn't. Mail server knows best, client ..... not so much!

There's always the possibility there's something along the way which is preventing delivery of the message. For example, our mail goes through our ISP and we have to enter into their portal all the valid email addresses we will accept mail for.
The email will hit our ISPs servers, which will check if the recipient is in the database of allowed recipients, and if not it bounces the message with a similar No mailbox by that name type NDR.

There could also be a hardware device on the clients side (spam filtering, antivirus etc) which is bouncing the message with some obscure NDR message.

What I suspect is most likely however, is that the email address really doesn't exist.

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Please make sure that you have enabled SMTP authentication in your Mail Client Software like Outlook etc.

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