spf.serverdomain.org has no MX records.
Question and response
Question
dig spf.serverdomain.org. MX
Answer
The response has no answer records.
Authority
serverdomain.org. 3600 SOA ns1.nshost2.de. hostmaster.webhostone.de. 2015062903 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0