mail.mainns.net has no MX records.
Question and response
Question
dig mail.mainns.net. MX
Answer
The response has no answer records.
Authority
mainns.net. 86400 SOA ns1.mainns.de. info.domains.resellerinterface.de. 1769416729 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0