[exim] unable to send mails to subdomains - dnslookup defer

Hi,

I am unable to send mail to someone@??? even though there is an exim server running on subdomain.mydomain.com.

I have two exim servers host1.mydomain.com and host2.mydomein.com. the mydomain.com is pointed to host1.mydomain.com. From a mail client hooked to dovecot of host1.mydomain.com, a mail to someuser@??? fails with error:

May 18 12:39:05 host1 exim[31857]: 2019-05-18 18:09:05 1hRycC-0008Hp-Bg <= someuser@??? H=([192.168.1.101]) [202.153.45.33] P=esmtpsa X=TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=no A=dovecot_plain:someuser S=1483 id=3AFA5894-E4E7-481E-BC5C-AC3CC5AD84C7@???
May 18 12:39:05 host1 exim[31859]: 2019-05-18 18:09:05 1hRycC-0008Hp-Bg == someuser@???=dnslookup defer (-1): host lookup did not complete

... Necktwi