Backup Mail Server Questions

[Brent Wiese]

> That's the hard part.  The Secondary MX'ing part is fairly easy.  All
> you do is get your friend to add an MX record to the DNS
> 'yourfriend.com' zone listing your server as a high numbered MXer:
> 
>     $ORIGIN yourfriend.com.
> 
>     @       IN    MX   0 smtp.yourfriend.com.
>                       10 smtp2.yourfriend.com.
>                       30 smtp.you.com.
> 
> And then add:
> 
>     yourfriend.com    RELAY
> 
> to /etc/mail/access and rebuild access.db.  That means your machine
> will accept e-mails addressed to users @yourfriend.com and queue them
> up for relaying onto to his servers as soon as they come back up
> again.  If his site has to go down for an extended length of time, you
> can make special arrangements to store incoming mail for longer than
> the usual 5 days and then flush it over to him when he comes back up.

I have a question that builds off this.

Is there a way to make the backup MX server understand that some mail is
ultimately destined for it and try to deliver it locally?

Here would be an example:

Mydomain.com is MX'd to mail.mydomin.com, which handles email for all my
users. On that server, I've set up an alias for support@ that is actually a
forward to my ticket system box (ie: support at tickets.mydomain.com).

In the event my main mail server is down, I'd like to use the
tickets.mydomain.com box as the backup MX. Its already running SMTP to
handle the tickets, so seems a logical choice. 

What would be ideal is to have mail destined for support@ to be delivered
locally. So, for example, a user can create a ticket saying the mail server
is down (of course that is only useful if admins have off-site email
addresses the ticket system notifies for redundancy, but that's easy
enough).

Mostly interested in knowing how to do this under Postfix, but I'm not
married to it.

Thanks,
Brent