Multiple IP addresses for same host name in /private/etc/hosts?
Unfortunately, the answer is no. The hosts file is a static lookup file, and because of this it has no concept of multiple-address per name failover, round-robin, or other features baked into the name resolver.

You can specify multiple names for a given IP address, but not multiple IP addresses mapping back to a single name.

Dare I ask, why can't you access the external interface of the mailserver while internal to your office? We configure all of our clients to use Mail/Jabber externally so they never run into this issue.

Despite conventional wisdom, using the external interface when internally does not "pull all the way across the internet", if configured correctly, of course.