Mailbox unavailable error

Question

When trying to send out an email in a .NET site, the following error is being encountered:

Mailbox unavailable. The server response was: No such user here

Does this error appear if the code is trying to send to an email address which doesn't exist?

Thanks.

I now have more information about this error. The emails are sent from 'noreply@[domain]'. When the emails are sent to an email address of the same domain, the emails are sent without a problem. This error only appears when the email addresses being sent to are not from the same domain. I don't know if that's any use?

Answer 1

This happens when you specify a domain with your NetworkCredentials. If you specify a domain (third argument) then you can only send to valid mailboxes within that domain. Leave it out to be able to send to any address outside the domain.

var client = new SmtpClient("smtp.server.com");
client.UseDefaultCredentials = false;

// The following will be able to send to anyone outside the domain.
client.Credentials = new NetworkCredential("user", "password");

// The following will only work when sending to users on server.com
client.Credentials = new NetworkCredential("user", "password", "server.com");

Answer 2

Could be that your password is incorrect. I had to reset the password on the mail server, then the error went away.

Answer 3

This may happen when you switch from 2.0 platform to 4.0. As it was explained here you need to tell IIS explicitly that you are not using default credentials and domain. Use the following syntax in web.config:

<network host="mail.younameit.com" port="25"
userName="[email protected]" password="youchoose"
defaultCredentials="false" clientDomain=""/>

The last two parameters are most important to fix this problem.