Why does mail _always_ fail with an “unknown host” error?
ANSWER 1: You have set up a mail client, with a subsidiary.cf, but you have neglected to define a mailhost. You can verify this with the following command: % ping mailhost corpmail2 is alive If the ping instead give you ‘host unknown’, you must define a mailhost before you can expect mail to work. ANSWER 2: You have set up a mail server, with a main.cf, but without following the precise instructions given in Section 3.1. As a result, your server is expecting to find a relay machine called ‘ddn-gateway’, and when it does not, mail fails. If you should have a mail relay (such as in the firewall situation described in Section 3.5), then define the DR and CR lines appropriately in your sendmail.cf. If you should not, then follow the instructions in Section 3.1 to make sure that all references to relays are removed from the sendmail configuration file.
