When you receive the error ‘503 Valid RCPT command must precede DATA’ in Outlook, this indicates an issue with your connection settings. It will prevent you from sending emails, but thankfully it is easy to fix.
First you will need open your account settings in Outlook. To get to this screen may be different depending on your version of Outlook, but generally you would click on Tools > Account Settings. You can then select your email account and click Manage or Change. Once you have the settings in front of you, click More Settings and then on the Outgoing Server tab.
Click “Outgoing Mail Server (SMTP) requires authentication” and make sure “Use same settings as incoming mail server” is selected. Then click on the Advanced tab and set the outgoing Port (SMTP) to 587. Finally, click OK and then Test Account Settings.
This should resolve the issue for you and allow you to send out mail without issue. If you require further information/assistance on this, please submit a ticket and a member of the team will be happy to assist further.