Email problem from default email account :: Horde

I am receiving below bounce back message when I try to send emails from my default email address (which CPanel creates automatically).

There was an error sending your message: Failed to send data [SMTP: Invalid response code received from server (code: 503, response: All RCPT commands were rejected with this error: Sender verify failed Valid RCPT command must precede DATA)]

I am trying to send emails from Horde interface. However, all the other email accounts work fine except this default one. Is there any idea what can be the problem?

Thanks,