This is a configuration problem with how your mail is set up.

It sounds like your mail server, smtp.myserver.com is only configured to accept mail for @myserver.com addresses and will not forward your mail to @xyz.com addresses. You will need to either talk to the xyz.com mailserver directly or reconfigure your own mailserver.

Note that relaying mailservers still are a major source of spam, so you need to be very careful with your setup when changing it to allow specific relaying.