RoseB said:
Receive "503 No recipient(s)" when trying to send email to valid addresses
using Outlook. What's wrong?
Read ftp://ftp.rfc-editor.org/in-notes/rfc2821.txt
Section 3.3, Mail Transactions.
Section 4.13, Order of Commands
So the 503 command is due to a bad sequence of commands. Turn on the
troubleshooting logging to see when the 503 error status gets returned, and
what commands were sent during that mail session before that point. Only
poll 1 mail account if you have multiple accounts; otherwise, you will have
a mix of commands listed from multiple concurrent account polls.
