This article explains you to fix 503 valid RCPT commands must precede DATA  in outlook. Mostly this error will occur without authenticating the SMTP on outlook.

To resolve this issue you need to follow few steps as mentioned below.

Step 1: Open your Outlook

Step 2: Go to File -> Account Settings -> Account Settings

Step 3: Choose the account that you want to reconfigure the settings, click change

Step 4: Click on More Settings, then click Outgoing Server tab.

Step 5: Click “Outgoing Mail Server” requires authentication and make sure “Use same settings as “incoming mail server” is selected.

Outlook settings

Step 6: Click on Advanced Tab, Set outgoing port to 587 or 465 or 25. As per specification is given by server end.

IMAP and SMTP settings

Step 7: Click Ok.

We hope this may resolve your issue!