2 questions about relay mail servers and secondary mail servers.

[Dan McCullough]

I hope everyone is having a good day or evening depending on where you
are, if I woke you I apologize.

I am looking to add redundancy and solve some ongoing mail issues.  My
first issue is that I want to setup a second mail server to act as a
fail over mail system, so that if the first one is unavailable there
is a small fail-over until the other one is fixed, turned on, plugged
in, whatever.  Now for some information I am using Fedora, Postfix,
Dovecot, etc for the mail server setup.  Also the system does not
store emails, except in the case that someone is out of the office,
but as a rule very few people have mail stored on the server.

Are there steps out there to follow?
Are there pro's and con's?
Any real experiences?

The other thing that I am looking to do is for our satellite offices
is installing small mail relay servers, that will allow internal staff
to relay mail, rather then connecting to the vpn and sending mail that
way.  One thing that I have noticed is that the smaller offices are on
DSL and have some issues when sending large > 5MB attachments, which
is a lot of their mail, and we will have issues where the mail server
here will not get all the data in time and drop the connection before
it has finished.  So my thinking was give them a relay mail server
that would send mail here, if it dropped it would continue retrying
until successful, unless I am missing the point.  However I have been
told that mail relay might be a problem with DSL connections as those
typically get labeled as spam since they are dynamic IP addresses,
technically our IP addresses are labeled dynamic even though their ISP
consider them static.

Any thoughts or suggestions?