First of all, please realize that if the domains in your post aren't the ones actually used in the test, if you changed them to keep us seeing what was really there, then there's no way anyone can help you.
According to your post, you tried to send mail to "
[email protected]".
mydomain.com is a domain owned by someone in Vancouver, Washington. Mail sent to that domain is handled by three mx servers:
mx 0 m1.dnsix.com
mx 1800 m.dnsix.com
mx 1800 ms1.dnsix.com
Since the first and last are the same, only the first two "count".
What this error is telling us is that you reached an mx server, either (probably) m.dnsix.com, or m.dnsix.com. And that the mailserver tried to find dns records for shell.myselldomain.com, but returned that shell.myselldomain.com wasn't in DNS as a valid domain.
Most mail servers, including the ones acting as mx for mydomain.com, won't accept email from servers it can't verify as existing. If your server doesn't exist it shouldn't be able to send mail, should it?
Spammers often use phony domain names; mailservers that block mail from nonexisting domains (and therefor nonexistent servers) do it in an attempt to keep spam traffic down.
If that's really the error message you got, then the test is faulty because it uses nonreal information.
If that's not the error message you got, then of course we can't help you unless you give us the real information.
Jeff