Yes, list it as lower priority in your MX records, and configure the RaQ's sendmail to forward all mail to your real server. In the event of a main server failure, it will accumulate your mail until your main server comes back up.
I would like Sendmail to be a backup for various domains that have different primary mail servers.
Is it possible to set it up in such a way that when the primary mail server is down and mail starts to arrive at the backup mailserver, it then looks up the MX records and try's to deliver according to the MX records, rather than having Sendmail conf'ed to forward mail to just one server?
Here is what you will need to do:
1. Config your backup MX to relay for the domain you want to backup. (/etc/mail/access)
2. Modify the backup MX sendmail config to allow use of the mailertable.
3. Pop your domain in /etc/mail/mailertable along with the final destination server.
4. Add your new backup server as an MX of lower priority in your DNS.
Now when your primary fails, your secondary will accept the email and then queue it up for delivery to the primary server. It will continue to try until your primary comes back online.
A word of caution - if you do not also carefully config your spam control, you are likely to be flooded with junk from the bots and hucksters who will intentionally send to the lower priority MX in hopes that your "backup server" will not have the kind of protections as your "good server". Believe me... it happens a LOT!
Network Services > Email > Advanced and add the domain/hostnames to the "Relay Email From Hosts/Domains/IP Addresses (optional)" section... dns adjustments will be required to so the world knows they can use it if your primary mx is not working...