[Exim] Redelivery of locally delivered mail

* Can I deliver mail locally and, if unread within 15 minutes,
retrieve it from the mailboxes and forward it to another server?

* Can I specify that, when mail being routed to another server
is undeliverable for 30 minutes, it should be delivered locally?

* How about segregating the mail, queued for a specific domain,
into a collection of lockable mailbox files, named by localpart?

* Any other ideas for ways to improve the situation below ?

[Pointers to the FAQ and sample configs would be fine]

Thanks, Alex.

Background:

Two servers, let's call them M and U to save typing.
M runs exchange for people preferring Outlook for mail service.
U runs exim (and pop) for people preferring anything else.
Both are listed as MX for the same domain for redundancy.

When everything is happy, the U box uses a combination of
fetchmail and an additional Exim router entry to ensure that
mail is pulled/pushed to each user's preferred service.

When the U box is down, all external mail will deliver to M
and people who normally use U can use Outlook if in a hurry.

When the M box is down, all mail for Outlook users is being
dumped into the Exim queue on U for delivery to M ... and
so is inaccessible to them in (for example) Netscape. BAD!