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

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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.

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