We have 1 webserver (cobalT0 and on mailserver with several domains.
all servers, including the linux server are behind
a FW with NAT configured.
When from a website on the cobalt we send email to an internal domain, via sendmail, it tries to connect for the external ip adres of th FW.
We added domains to the hosts and lmhosts files
telnet and ip works fine it uses the internal adres.
However sendmail still resolves using the public DNS.