[SATLUG] Backup SMTP Server (Secondary Mx) Postfix/Debian

[Bryan Scott]

Hello All, 

I have read lots of info on relayhosts for Postfix.

What I want to do is configure an offsite SMTP server to act as a
backup server and list it with a priority of >90 to have it act as a
backup MX.

Problem 1: Is it really as simple as I have read? Just add domain name
to relayhosts? Mod DNS to reflect existence of this new mx server. (It
just seems that I am missing something, or that I am missing some of
the built-in logic of the standard smtp server)

Problem 2: If the secondary smtp server holds mail til primary comes
back online, and I do not specify valid email accts on the backup,
will there be an attempt to deliver to primary then be rejected and
gone forever? <-- Preferred ) Or.. will they be rejected by the
primary and left to sit on the secondary and die when the TTL runs
out? <-- Not bad ) Or.. will they sit there (on backup) forever
because it thinks that it supposed to hold on to those mails because
it suspects the primary is down and they need to be kept until the
primary can accept them. My basic fear is that Spammers will mail to
all sorts of invalid accts, and they typically would not be accepted
by my primary smtp server, but the backup does not know better.

Any pointers, help, or links would be greatly appreciated.

Thank you, 

Bryan