If you are trying to send email from Outlook and get below error:

503 valid RCPT command must precede DATA

So you need to follow below steps to fix this error.

1.  Open outlook

2. Go to Tools/Email Accounts

3. Select the Account and click Change

4. Click on More Settings


5. Click on Outgoing Server Tab

6. Click “Outgoing Mail Server (SMTP) requires authentication” and make sure “Use same settings as incoming   mailserver” is selected.


Now try  to sending an email in Outlook using the domain account.