I've seen my maillog and it has entries like these:
Jul 30 00:51:02 server sendmail: 0000060000006DF64: to=<firstname.lastname@example.org>, delay=05:48:23, xdelay=00:00:00, mailer=esmtp, pri=900000, relay=xyz.com, dsn=4.0.0, stat=Deferred: Name server: xyz.com: host name lookup failure Jul 30 00:50:57 server sendmail: 0000060000007C280: to=<8542@xyz>, delay=03:00:53, xdelay=00:00:05, mailer=esmtp, pri=810000, relay=xyz.com [x.y.z.d], dsn=2.0.0, stat=Sent (OK id=1V3y9S-0003zn-Fx)
So If I send 100 emails (all auto-generated and not spamming people), ONLY few have been "Sent" and a majority of them are "deferred". The mailboxes exist and the host is reachable with valid MX record.
What is the problem here?
Thank you for time and patience,