Emails sent to a particular domain (telnet-bd.com) is always using the lower priority MX which happens to be down and so the emails are not getting delivered.
From my server:
[root@mx1 ~]# host -t mx telnet-bd.com
telnet-bd.com mail is handled by 50 mx-in-02.telnet-bd.com.
telnet-bd.com mail is handled by 10 mx-in-01.telnet-bd.com.
[root@mx1 ~]# mailq
/var/spool/mqueue (18 requests)
-----Q-ID----- --Size-- -----Q-Time----- ------------Sender/Recipient-----------
m887bl45020906 704 Mon Sep 8 13:38 firstname.lastname@example.org
(Deferred: Connection refused by mx-in-02.telnet-bd.com.)
/var/log/maillog shows no connection attempt made to the higher MX mx-in-01.telnet-bd.com at all.
I can connect to the higher priority MX but not the lower 1 from the server:
[root@mx1 ~]# telnet mx-in-01.telnet-bd.com 25
Connected to mx-in-01.telnet-bd.com (18.104.22.168).
Escape character is '^]'.
220 mx-in-01.telnet-bd.com ESMTP server of Telnet Communication Limited (Telnet). Sending Unsolicited Commercial or Bulk E-mail or SPAM to/from TELNET Computer Network is Strictly Prohibited. Any Attempt Of This Type Of Abuse Will Cause From Termination Of Network Connectivity Up To Activity Of Law Enforcement Agencies.
221 2.0.0 Bye
Connection closed by foreign host.
[root@mx1 ~]# telnet mx-in-02.telnet-bd.com 25
telnet: connect to address 22.214.171.124: Connection refused
telnet: Unable to connect to remote host: Connection refused