Split from this thread.

The IP address you're trying to send from when this error happens is dynamically allocated from a block that SORBS has blacklisted. You can usually get round the problem by authenticating yourself to the server: Right-click on the account name in the folder pane and select Properties. On the Servers tab, ensure that My server requires authentication is selected. On the Advanced tab, change the outgoing mail port (SMTP) to 587.  Try sending a test message to yourself, then post back with the new error message if any.

I tried that for days, but can't get port 587 saved. It always reverts to port 25 immediately, I don't think  Microsoft has any business telling customers what they can or can't do for email. I even checked with my server (CentruryLink) and technicians but got a different answer from each one. Is there an easily understandable fix or resource where I can check? Thanks