Sounds like it could be an SMTP authentication issue.
You could try checking that your email account is set up to provide the same password to send messages (SMTP) as to receive them - this will be in the Outlook account settings.
Also check to see if SSL is set "on" for SMTP - if so, turn it off.
If it isn't that, try googling "503 No recipient(s)" - there's a variety of solutions out there!
SD