#!/bin/bash
awk -F\@ '{print $2}' mails.txt | sort | uniq -c | sort -nr > domains.txt

while read domain; do
    result="$(dig $domain MX +short | tail -n 1 | awk '{print $2}')"
    echo "$domain => $result"
done < "domains.txt"


=== result [45] ===
11 gmail.com => gmail-smtp-in.l.google.com.
8 yandex.ru => mx.yandex.ru.
2 ya.ru => mx.yandex.ru.
2 mail.ru => mxs.mail.ru.
1 xoyo.ru => mx.yandex.ru.
1 ukr.net => mxs.ukr.net.
1 tut.by => mx.yandex.ru.
1 torba.com => mx.yandex.net.
1 tns.im => smtp2.mailfence.com.
1 theaqua.im => in1-smtp.messagingengine.com.
1 test123.ru => test123.ru.
1 schizo.name => aspmx4.googlemail.com.
1 ro.ru => inmx.rambler.ru.
1 require.pm =>
1 playact.net =>
1 pascalhp.net => mx.yandex.net.
1 oleg.so =>
1 nm.ru => mx.qip.ru.
1 ngsv.ru => mx.yandex.net.
1 mail.lv => mx2.mail.lv.
1 lepus.su => aspmx2.googlemail.com.
1 innoultra.ru =>
1 evilsprut.com => in2-smtp.messagingengine.com.
1 dublin.com => mx01.mail.com.
1 default.im => mx.yandex.ru.
1 bk.ru => mxs.mail.ru.


== work mx [41] ==
13 google.com
16 yandex.ru
3  mail.ru
2  messagingengine.com
1  mail.com
1  qip.ru
1  rambler.ru
1  test123.ru
1  mailfence.com
1  ukr.net
1  mail.lv


== not work mx [4]==
REGISTERED  innoultra.ru
FREE        require.pm
FREE        playact.net
FREE        oleg.so