I've been hosting my own email on this domain for just over a year now and I hadn't noticed any problems until a couple of days ago a German friend of mine using GMX tired to send me an email and it bounced back with the error message "no valid mx hosts found".
It turns out that according to RFC 2181
a MX resource record must not be an alias.
I had mail.xo.tc setup as my mx record
And then because everything is running off this one server I had mail as a CNAME.
I deleted the CNAME and added in an A and AAAA record and that fixed the issue.
Now I can receive emails from gmx.de and I'm RFC compliant.