Using Sendmail version 8.13.1 from a Linux box.
Seem to be getting " lost input channel " and a "User Unknown " almost immediate. Can send email to every other external domains i.e @hotmail.com or @gmail.com but if the destination domain is the company's domain then it gets rejected.
This sendmail in on an internal network and allows mail out /in and due to other mail going out we know the firewall is not to blame for rejection. The mails being sent go via the correct SMTP server as seen in the mail log.
I have added a RELAY to this domain and even SPAM CONTROL for that Domain to ALLOW . But every time we get "lost input channel" and almost immediately the <email@example.com user Unknown then the "lost input channel".
It is really weird as to why we are unable to send to every other domain than our own.
Appreciate as to why .