As I remarked upthread, an MX record isn't capable of containing
anything but a FQDN, so the second item must really mean something
else (like "in a nonexistent TLD").
$ dig -t mx tester.hovland.be
[...]
tester.hovland.be. 8064 IN MX 5 mail.
[...]
# ping -ainet6 mail
PING mail.hovland.be (::1): 56 data bytes
So mx for tester.hovland.be points to the mailserver of whatever
domain your machine is under as long as the hostname
mail.yourdomain.com exist.
No, it doesn't. It points to the (nonexistent) top-level doamin
"mail".
It may be misinterpreted as being the machine "mail" in some domain or
other, depending on how carelessly it is handled (such as by stripping
the trailing dot and passing the rest to ping), but that is definitely
a *mis*interpretation.