> - is this what is called an "MX backup"?
Yes, this is what MX backups are for.
> - is there a way for his box to send me a request for a queue run as
> soon as its connection is re-established?
He could call you and start a script to force delivery. Maybe
kick_exim.sh as the initshell in /etc/passwd?
> - is there anything more to be done? how would exim handle the following
> situation: a domain called domain.com has two 'MX backups' like in..
> @ IN MX 10 mail.domain.com
> @ IN MX 20 mail.friend-a.com
> @ IN MX 20 mail.friend-b.com
> if mail.domain.com is unavailable, then mail.friend-a.com has to know
> that it should wait for mail.domain.com to come up instead of sending
> messages to mail.friend-b.com thus creating a loop!
Yes, indeed. Every MX delivers to lower numbers only. For DNS, I always
recommend the book "DNS and BIND" (O'Reilly).
