Hello,
If you are facing an error "503 valid RCPT commands must precede DATA" when you try to send an email. Some common solutions that might be helpful to resolve this error:
1) Apostrophes - Remove any apostrophes (single quotes) or other non-alphabetic characters from your email account name(s).
2) SMTP Port - Use Port 26 (Instead of the default 25) for outgoing mail OR (TLS-587 OR SSL-465)
3) Server Authentication - Set up your outgoing mail to use outgoing server authentication (sends mail by logging in with your email address and password).
In Outlook, Follow these steps below:
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 mail server" is selected.
Now, try sending an email in Outlook using the domain account.
4) Outlook PST files - Sometimes the 503 error will occur if your Outlook PST file is corrupted. Try repairing the file. (Before repairing PST file, take a backup at safe side)
For more information, refer this source URL: https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_other/503-valid-rcpt-command-must-precede-data/c9f20070-7021-460b-adda-b1cfe81b8162
