Status 5.0.0 is "Can't deliver. Don't know why. Can't retry, either."
Possible reasons:
- Your mail server's hostname (mailname) is not a valid FQDN.
- Your reverse DNS is not a valid FQDN.
- The other mail server is just being an arsehole.
- Your domain SPF does not allow your server to send mail. (Maybe you have conflicting records, or the DNS change hasn't propagated yet?)
Can't say anything more specific unless you tell us your Linode's hostname, the domain in question, and which company your other e-mail account is hosted with. If you need to keep this info confidential, try
Linode support.