We have a customer who's unable to deliver email to us, no one else that I'm aware of is having this issue. They're mail provider is hostgator.com and I've worked with their support staff to try and resolve and we aren't making any headway. We have an inbound spam filter and the logs do not indicate a connection attempt from abc.com, nor do the firewall logs show a connection attempt - watched them real-time. I have verified on both the firewall and spam filter their domain and IP address is not being blocked.
A traceroute and DNS lookup from their mail server work properly. I have verified our domain is clean, not on any blacklists. Below are the logs from their attempt to send email to us from trouble domain.
2014-02-18 14:51:46 1WFrdq-00072o-So <= firstname.lastname@example.org H=localhost.localdomain (220.127.116.11) [127.0.0.1]:60990 I=[127.0.0.1]:25 P=esmtpa A=dovecot_login:email@example.com S=638 firstname.lastname@example.org T="gatoraustin2" from <email@example.com> for firstname.lastname@example.org
2014-02-18 14:51:46 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc 1WFrdq-00072o-So
2014-02-18 14:51:46 1WFrdq-00072o-So SMTP connection outbound 1392756706 1WFrdq-00072o-So abc.com email@example.com
2014-02-18 14:52:50 1WFrdq-00072o-So mail2.twns.com [18.104.22.168] Connection timed out
2014-02-18 14:52:50 1WFrdq-00072o-So == firstname.lastname@example.org R=lookuphost T=remote_smtp defer (110): Connection timed out