I came across an issue following a new firewall installation whereby Sendmail was unable to access the mail server on the internal network. Due to having no control over the DNS my initial solution was to add the internal IP to the /etc/hosts file however Sendmail didn't seem to take much attention to it. After a little research it turned out that by default SendMail uses DNS for name resolution, not the /etc/hosts file. This post outlines how to resolve this issue.
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