Thanks for commenting. It appears that all my mail is routed through 'yahoo'; that is how Rogers mail seems to be set up. But my mail goes through fine if I'm use Rogers web mail, so it can be a rejection from Yahoo. That is how Rogers determined that it must be Outlook, because the mail goes through fine when I use webmail instead of outlook.

It takes about a day for the rejected message to get returned to me. It stays 'in the queue' a long time. I really hate the thought of abandoning Outlook because of this, but I'm running out of ideas to try.

Any other thoughts or information I can provide to troubleshoot this?