I understand about callforwards, but it seems to me that the backup MX
is mostly going to get mail when the main MX isn't available, so I
don't see how this would work very well in practice.
Actually, it works pretty well in practice. The main problem with secondary
MXes is that spammers send messages to them even when the primary is up (which
in most cases is 99+% of the time).
This leads to the fairly obvious question: if the primary is up 99+% of
the time, what's the point of a secondary? There was an era when they
were useful to deal with networks that didn't have routes to some parts of
the net, but I'd think that's a pretty small niche now. Unless your
primary is down for a long time, days or longer, any legitimate mail will
just wait and retry and get delivered anyway.
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Cambridge UK
"I dropped the toothpaste", said Tom, crestfallenly.