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!