mixer.unisender.com has no MX records.
Question and response
Question
dig mixer.unisender.com. MX
Answer
The response has no answer records.
Authority
unisender.com. 3600 SOA ns1.yandexcloud.net. mx.cloud.yandex.net. 1 10800 900 604800 900
Additional
. 0 OPT ; payload 1024, xrcode 0, version 0, flags 0