Seems this topic has hit a bit of a nerve and various people (myself
included) have put up excellent cases for and against having secondary
mail servers. What it boils down to is who's mail servers you trust to be
reliable enough which includes being able to poke & prod them after
something has gone wrong somewhere. Below I discuss these hopefully in an
unbiased manner.
[sender]----------->[receiver MX]
or
[sender]----------->[backup MX]------->[receiver MX]
Any one or more of the above may be on semi-permanent links to complicate
the situation. In a corporate situation both the receiver and backup MX
server(s) should probably be under control of the same department.
A reality check here says that you can't always trust the sender's system
to handle retrying of the email. Granted, most will handle it, pretty much
all unix based mail systems I've seen are capable of this. Who's to say
that some company hasn't got a crap system.
Another point raised is that the sender or the receiver may be on a
slow/intermittent link, furthermore it is possible that there are a
significant number of network hops and packets may not even be able to
travel between sender & receiver directly (see discussions on IP, icmp &
TTL) or indeed internet routing is temporarily screwed - it happens
regularly.
Choice of a suitable backup MX (if you go that way) is a key issue, it
really should be more reliable and better connected (ie ISP peering) than
your own to be of benefit. I've seen our LinuxHelp mail server online and
accepting mail from some places whilst other emails go to the backup MX
server due to network issues beyond our control. In this case it was
FASTER to have a backup MX server. Again in our case I have root access to
both mailservers. If you don't have sufficient access or have a contact
that does and is reasonably available then it is arguably less value to
you.
Another use of local MX servers is load balancing and redundancy in a
business situation where the company pretty much relies on having email
up. You can then take one server offline for maintenance whilst the other
is chugging along.
In my opinion, on the whole, for personal use I probably wouldn't
recommend the MX unless you're capable of managing it. For a business I'd
definitely recommend it and as a requirement have a competent sysadmin for
it - whether that be one of yours or outsourced.
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