ns4ac.portunity.com has no MX records.
Question and response
Question
dig ns4ac.portunity.com. MX
Answer
The response has no answer records.
Authority
portunity.com. 360 SOA nset1.portunity.com. hostmaster.portunity.de. 1710232264 10800 3600 3600000 360
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0