Originally posted by lundy
Make sure that your outgoing mail server (SMTP server) is set to your ISP's mail server.
E.G. if you use Comcast, the SMTP should be "smtp.comcast.net".
That wasn't the problem, but you gave me an idea. I checked through the "Account Information" (where smtp is shown), then to 'Server Settings'.
Under 'Server Settings Options', "Authentication" showed:
'MDS Challenge' instead of 'Password'. I changed it to read 'Password' and Mac Mail Out began working again.
Thanks for the clue. And thanks to other members for their suggestions.
This was my first time using AppleInsider and it was a very pleasant experience. I switched from 25 years with PC's to Mac a short time ago and felt ALONE. AppleInsider made me feel 'connected again'. A great feeling.