I have 2 email addresses, A and B.  I forward emails from B to A, but send out emails from A, using B as the alias.  That way I don't have to keep logging in and out.  This all worked perfectly on Hotmail, and worked Outlook until about 3 weeks ago.  Now every time I send an email from A, but choose my alias (B) I get the email returned with this message:

"This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed."

....."

It used to work; I can't work out what could have changed to stop it working.