Hi, I haven't been able to find an answer to this problem. I am running WHM 9.9.8 cPanel 9.9.8-R119 of WHM/cPanel and I added a new account, then I renamed the domain of the account to something else. Now when another account on the server tries to send mail to the domain that I initially set up, but changed, it results in this error:

Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: 503 valid RCPT command must precede DATA

I looked up this error and found that it often occurs because of POP before SMTP or SMTP Authentication problems but mail relayed through the server to other domains works fine. The domain that I initially set up is hosted on another server and DNS is properly pointing to that server.'

Any ideas?