You want to configure a backup mail server for a mail destination.
Add an MX record for the mail destination, listing the domain name of the backup mail server at a higher preference value (and therefore lower preference) than the main mail server. For example, say the MX record for the mail destination foo.example, currently looks like this:
foo.example. IN MX 10 mail.foo.example.
To add smtp.isp.net as a backup mail server for foo.example, add another MX record to foo.example pointing to that mail server, with a preference value higher than 10. For example:
foo.example. IN MX 20 smtp.isp.net.
By default, most modern mail software won't accept mail addressed to just any old domain name. You may need to configure the backup mail server to accept mail addressed to the mail destination. But don't configure the mail destination as local to the mail server; the backup mail server should accept the mail but queue it for later delivery to your main mail server.
4.2.4 See Also
Section 2.5 for instructions on setting up a mail destination and "MX Records" in Chapter 5 of DNS and BIND.
BIND Name Server Configuration
BIND Name Server Operations
Delegation and Registration
Interoperability and Upgrading
Resolvers and Programming
Logging and Troubleshooting