553 sorry, that domain isn't in my list of allowed rcpthosts

Here's an annoying little error that I've just come across.

I've tried to send an email from Outlook and I've received a reply from "System Administrator" with the word "Undeliverable" in the subject field along with the subject of the mail I tried to send.

In the message itself I get the following:

Your message did not reach some or all of the intended recipients.
   
    Subject:    Out TEST 1
    Sent: 09/10/2009 14:49

The following recipient(s) cannot be reached:

    'mtminnock@gmail.com' on 09/10/2009 14:49
        553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)

The message is a little bit cryptic as the idea of a list of allowed recipients indicates that there's a list of disallowed recipients and that might lead you to believe that your firewall or anti-virus systems is acting up and doing more than it's supposed to.

What's actually happening is that Outlook is trying to tell you that your outbound email settings are wrong. In my case it was the "Outgoing mail server (SMTP)" setting that was wrong and this had been caused by my changing Internet Service Providers. My new ISP has a different SMTP server and I needed to tell Outlook what this servers name was.

In order to change the SMTP setting for a given e-mail account in Outlook you need to do the following:

1. In Outlook go to Tools > Account Settings. Select the account you want to fix and click on Change.
2. About halfway down the window that opens is a section headed "Server Information". In the third box down enter the correct name of the SMTP server (this can be obtained from your ISP). Click Next and Finish and that's it!