You probably want to analyze your email server needs. Most of the small shops I'm familiar with, only run a single server. First off, down time is usually rare and limited to a day (or less) when it happens (assuming you have a good install/config documentation, backups, and a recovery plan). Second, most email servers try for a day or three before bouncing the email, and most when they finally bounce will send a message back to the sender. Third, people are familiar with how email works, so a bounced mail just means they send it again, and a missing email is relatively common (lost in transmission, caught by some filter, etc) that if it's important they'll try another means of communicating. All that adds up such that a second backup email server is usually a waste of equipment/sysadmin time/money.

A good example is several of my associates turn their lab email servers OFF over the weekends. Some data (although I'm not yet convinced) seems to point to way less spam, because the spammers think it's a bad address (they don't resend ever).

So determine the value of the email your server (and users) deal with - then figure out if that's worth a second server (and remember that if you decide to run a second email server, it needs to be in a completely different data center then the first).

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Either provide enough details for people to help, or sit back and listen to the crickets chirp.
Security thru obscurity is a myth - and really really annoying.

Thanks to all for all your answer