[Exim] Long time down MTAs

Using Exim 2.11 as I understand it when an MTA has gone down
exim will use the retry rules to continue to deliver mail until the
retry rules expire. At this point exim will give up on any mail for that
MTA and stop accepting incoming mail for that customer.

How does Exim know that the remote MTA has come back up. I
guess that a successful delivery will flag the MTA as available
again. However if the exim system it secondary MX for the MTA,
once the MTA comes back up no further mail will arrive. Does the
"flag" that says an MTA is unavailable expire should no
unsuccessful delivery attempts be made.

Cheers,

Martin Stewart