Quote from moshe on 11/05/2021, 4:28 am

Problem

When trying to use sendmail on linux systems to send simple mail user gets the following error response from remote mail server

450 4.7.1 <localhost.localdomain>: Helo command rejected: Host not found
SMTP recipient() command failed:
4.7.1 <localhost.localdomain>: Helo command rejected: Host not found

Solution

Configure sendmail verified domain

install sendmail and utils

yum -y install m4 sendmail sendmail-cf.noarch

Configure sendmail with verified domain

vi /etc/mail/sendmail.mc

add to the end of the file

define(`confDOMAIN_NAME’, `server.tracston.com’)dnl

m4 sendmail.mc > sendmail.cf

service sendmail restart #( if its running as a service

add to your /etc/hosts

your verified fully qualified domain example:

127.0.0.1 server server.tracston.com server
192.168.10.10 server server.tracston.com server