It is possible to set up multiple smart hosts in Sendmail. The syntax required in the /etc/mail/sendmail.mc file is:
The square brackets are to ensure that Sendmail does not do a Domain Name Service MX lookup. After editing the /etc/mail/sendmail.mc file, the /etc/mail/sendmail.cf file must be regenerated.
For Red Hat Enterprise Linux 2.1, use:
m4 /etc/mail/sendmail.mc > /etc/sendmail.cf
For Red Hat Enterprise Linux 3, use:
make -C /etc/mail
Note: If you are running a Domain Name Server (DNS), an alternative to specifying multiple smart hosts is to set up your DNS server to have multiple MX records under the smart host's host name and have the DNS server 'round robin' the MX records. This configuration, however, is out the scope of this Knowledgebase article