Symptom: Client using Outlook reports that sending mail fails with an undeliverable notice. The error message in the undeliverable notice indicates a 503 (No valid recipient/recipient must precede data) error.
Cause: Outlook settings must be modified to authenticate when sending.
Resolution: Select Tools -> Email Accounts -> View or change existing email accounts -> Select the affected account -> Change. On the account properties screen, choose More Settings -> Outgoing server. Enable the option "My outgoing (SMTP) server requires authentication". Generally, if the client is checking mail and sending it from a created POP account, leaving the first option ("Use the same settings as my incoming mail server") is sufficient. If the user wishes to send mail under a different login, change to the "Log on using" option and input the appropriate username and password. Note that for all secondary email users, the login name is the full email address of the user. Source