validrcptto is a qmail plugin which verifies that the email address that you're sending email to is valid. There's a list of valid addresses that the qmail server can handle, and it parses the SMTP conversations RCPT TO: line against that list. So somewhere, there is a qmail server refusing to accept mail for the user.
You are getting the response from your email server, telling you the error it received during its conversation with the broken qmail server. Contact that server's admin and ask. Or verify the spelling of the email address you're sending to.
-tom