Dead.letter email. Same domain name, different mail server

I have a server setup "myserver.com" and I'm trying to send emails from a PHP/Apache application on this server. The host name is set 开发者_如何学Pythonto "myserver.com" and I can send emails to anyone outside of the myserver.com domain.

We are using a separate server/service and the problem is when I try to send emails to anyone on the myserver.com domain they don't get the email.

This also happens when I try to send via command line, I get a: dead.letter... Saved message in /home/username/dead.letter

It seems that the MX record is being ignored... how can I force it to respect the MX record?

Thanks.

how can I force it to respect the MX record?

By setting a proper hostname. If the server in question does not provide every single service for myserver.com, it needs to use a subdomain.