Error: undeliverable: Server error: '550 5.1.1 relaying not allowed
this issue just began. I simply reply to an email address I have been for years with the same mail client Outlook 2010 and out of nowhere the message immediately gets returned by the System Administrator say Server Error undeliverable: Server error: '550 5.1.1 relaying not allowed'
Note, there is no hexadecimal error code.
Solution: in Outlook go to the Account Settings (which should be off the File Menu though there are many version of Outlook and the version you have may be different if so use the question mark or help menu to search account settings), then select the Cox account having issue. Choose "Change" then "More Settings". Look for the tab that says "Outgoing Server". At this tab check the box that shows MY Outgoing server requires authentication". The radio button below this "Use Same Settings..." will also be selected. Click OK then Next and the test message should complete in both directions incoming and outgoing successfully.
The reason this has occurred is due to Cox cracking down on Spam Mail. By including these settings it tells the Cox server you have an account with a proper password. If you call Cox support they will insist its an Outlook issue despite the Server Message