Server Error: 550, Error Number: 0x800CCC79

[Gerry]

I have been fine sending emails for a while, and since
yesterday I have not been able to send out emails - only
receive - I get this error message on all my accounts:

Server Error: 550, Error Number: 0x800CCC79

What does this mean?

 

[Jeff Stephenson [MSFT]]

Your server is saying that it can't send to some of the recipients in your
message. My guess is that yesterday your mail provider turned on an option
to require that you authenticate to their server before you're allowed to
send. Go to the "Outgoing Server" tab on your account to set up
authentication to that server.