I get "503 valid rcpt command must precede data" when I try to send email via Mail App in Windows 10
Follow the steps below:
1) Open Mail App
2) Go to Settings >> Manage Accounts
3) Select the Account and click Change Mailbox Sync settings
4) Click on Advanced Mailbox Settings
5) Click on "Outgoing Server Requires Authentication" and ensure is selected
6) Click "Use the same User name and Password for sending email" and ensure is selected.
7) Click Done
8) Click Save
Now try sending an email in Mail App using the domain account.