SA Gateway -> MS Exchange -- what if MSE down?

[Tony pace <to...@owfg.com>]

we are looking to implement SA in our environment this best describes 
what we want to do.

[SPAM/HAM] --> [ SA GATEWAY] -----------------------------> [MS EXCHANGE]
		- system wide filtering			- all user mailboxes
		- postfix transport			- MX SEC RECORD
		- MX PRI record			

the question that was posed --- if  the MS Exchange is not accessible (network 
issue, down for maintenance) -- what happens to the email?


My best understanding is the email will be rejected as "mail-server not 
available", as SA is a filter not an MTA and that Postfix is a check/forwarding 
agent (not "store" & forward).


Would I be correct in assuming, in the event that if MS Exchange was down, in 
order to "store" mail -- I would need to have a backup MTA with all the users 
mailboxes replicated?

Thanks,
Tony