554 5.0.0 MX list for example.com. points back to server1.example.com

Issue

• MX list for example.com. points back to server1.example.com

   ----- The following addresses had permanent fatal errors ----- <username@example.com>

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554 5.0.0 MX list for example.com. points back to server1.example.com
554 5.3.5 Local configuration error

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451 example.com: Name server timeout
Warning: message still undelivered after 4 hours Will keep trying until message is 5 days old

$ grep -i "no route to host" /var/log/maillog | wc -l
268

Mar 16 12:23:19 server1 sendmail[417]: t2GFNHWC032677: to=<username2@yourdestination>, delay=02:00:01, xdelay=00:00:00, mailer=esmtp, pri=211709, relay=your.relay.server.com., dsn=4.0.0, stat=Deferred: your.relay.server.com.: No route to host
...

$ grep -i "host unknown" /var/log/maillog 
Mar 22 09:11:00 server2 sendmail[3167]: t2M5CBKA027758: to=<syslog@example.com>, ctladdr=<root@server2.example.com> (0/0), delay=08:58:49, xdelay=00:00:00, mailer=esmtp, pri=1691182, relay=example.com, dsn=5.1.2, stat=Host unknown (Name server: example.com: no data known)
Mar 22 09:11:00 server2 sendmail[3167]: t2M5CBKA027758: t2MEAxNX003167: sender notify: Host unknown (Name server: example.com: no data known)
Mar 22 09:11:01 server2 sendmail[3167]: t2L5CqeX023454: to=<syslog@example.com>, ctladdr=<root@server2.example.com> (0/0), delay=1+08:58:09, xdelay=00:00:00, mailer=esmtp, pri=6338097, relay=example.com, dsn=5.1.2, stat=Host unknown (Name server: example.com: no data known)