Why does mail _sometimes_ fail with an “unknown host” error?
ANSWER 1: This can occur on mailhosts prior to 2.5 if you neglected to install the sendmail.mx binary. Compare the size of the sendmail and sendmail.mx binaries: % ls -l sendmail sendmail.mx -r-sr-x–x 1 root 205846 Mar 20 1995 sendmail -r-sr-x–x 1 root 234708 Mar 21 1995 sendmail.mx If they are not the same size, you are not using sendmail.mx, and thus sendmail will generate “host unknown” errors on any attempts to mail to an MX record. Follow the instructions detailed in Section 2.1 to replace the sendmail binary with sendmail.mx. ANSWER 2: You do not have the “OI” entry in the /etc/mail/sendmail.cf file because you just copied over one from an another machine (typically seen with upgrades to 2.5 and 2.5.1).
