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 … Continue reading How to Solve “Error 503: Valid RCPT command must precede DATA” (Outlook 2016)