mailgw.mikrotik.com has no MX records.
Question and response
Question
dig mailgw.mikrotik.com. MX
Answer
The response has no answer records.
Authority
mikrotik.com. 3600 SOA dns-master.mikrotik.com. support.mikrotik.com. 1772531162 3600 900 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0