The no relays message normally means that the email client is not configured properly.
For instance if someone switches from Freeserve Dial-up to be BT OpenWorld Broadband but are still using their Freeserve email. If they try to send an email using the freeserve server it will be bounced with a no relay as it is a measure (of sorts) to stop spamming. In this case they would need to switch the outgoing server to mail.btinternet.com for it to work.
If on the other hand they get the email address wrong then an appropriate message should be sent back - like domain or user not found.
hth