When is the /etc/hosts file loaded

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.

Nothing will break (or hasn't for me whenever I resort to this on occasion) - you'll have to remember to re-do the permissions if and when the OS updates that file systematically. You'll also want to watch the permissions if you ever "repair permissions" since that file is likely recorded in a system receipt. I'd go so far as to set a launchd task to daily enforce your "custom" permissions if you feel it's worth that much work and log something to the console about the need for this "adjustment"

This is a far faster and simpler (and therefore probably better) solution to setting up your own DNS or NIS even though a trip down memory lane would be fun for a few moments:

• cd /var/yp
• make