Re: [exim] Is a secondary MX worth the effort?

I don't run with a secondary MX, though from time to time I think about the same sorts of things you outlined. Here are the two
reasons I've considered using a secondary MX (though neither has motivated me to actually do it):

1. When your primary comes back up, you can poke your secondary to have it immediately send all the accumulated messages. If
you instead rely on retries from scattered senders, there will be additional delays in their queues while they are waiting to
retry. Getting messages grossly out of order can be annoying to users.

2. Some sending hosts are completely retarded about retries in the event of a failed primary. I don't know if it's still true,
but the mailer for Yahoo Groups used to immediately unsubscribe recipients if a single message was attempted for a failed
primary (presumably they would try secondaries if they exist, but I don't know). I used to have to spend a lot of time helping
users recover from this after routine half-hour maintenance windows, etc.

One of the things I've considered for #2 is having a secondary MX which is configured to give back a 4xx for everything.
However, I don't know if there are sending hosts who would fail to go back to the primary MX when it came back up.