What does “503 valid RCPT command must precede DATA” mean?
If you have tried sending email but receiving an error with the following message: “503 valid RCPT command must precede DATA”, then you need to configure your email client (ie Outlook Express) to authenticate automatically.
In outlook express, you may go to “Tools >> Accounts >> Mail >> Properties >> Server”
Check “my server requires authentication” box in your email configuration.
In most programs, this is in advanced settings and on a Server settings or Outgoing server settings.