Error 503: Valid RCPT command must precede DATA

[66carolin]

May I know the reason for Error 503: Valid RCPT command must precede DATA?

[Carol.P]

The most common reason for this error is an incorrectly configured mail client.
In order to solve this issue you should reconfigure your email client to authenticate to the SMTP server.
In Outlook, for example, this can be done by checking the "My server requires authentication" box in the Outgoing Server section of your mail options.