Answering online query

Hi,

The key error is:
503 Valid RCPT command must precede DATA

This is an SMTP protocol error meaning the mail server rejected the email because the RCPT TO (recipient) command wasn't properly sent before the DATA command. In other words, the server doesn't know who to deliver to before the message body is being transmitted.

Common causes:
1. Invalid or empty recipient email address: The "reply to" address from the contact form query might be malformed or missing, so the SMTP server has no valid recipient.
2. SMTP server authentication/configuration: The Joomla mail settings (Global Configuration > Server > Mail) may be misconfigured: wrong SMTP host, port, or authentication credentials.
3. SMTP server rejecting the sender: Some servers require the sender domain to match the authenticated account, and reject the sequence otherwise.
From what you're saying, it seems that the email address of that user is just invalid.