The “Error 503: Valid RCPT command must precede DATA” happens when sending email. Outlook is telling you that your SMTP (outgoing mail) connection was not properly authenticated by your email server.
The two most common causes for this error are:
- Your email server requires you to check email first before sending email (checking email first is one way your email provider manages the security of your email account).
- Your email client isn’t set up for SMTP Authentication.