[Exim] Retry rules/failing host

Hi all,

I have a feeling I'm missing something obvious here, but I keep looking at
my config file, at the docs, and back again and I still can't work out
what's happening.

I've got a server (Exim 4.10, Linux) which is backup MX for a domain. The
domain's primary MX has got network problems (routing loop I think) and is
therefore inaccessible. The messages are correctly ending up on my server,
but they seem to be getting kicked off the queue too soon.

The retry rules are as default:

* * F,2h,15m; G,16h,1h,1.5; F,4d,6h

However, messages seem to be bouncing after only two days. For example, a
message received by the server for user@??? at 05:48 on
Saturday morning caused a permanent error message to be generated at 05:48
this morning, with "No route to host: retry timeout exceeded".

Also, the errors seem to vary between the one above, and "retry time not
reached for any host after a long failure period". Am I right in
understanding (from section 31 of the spec) that the latter message is
generated where each time a delivery for the message in question is
attempted, the retry time hasn't been reached (because preceding messages
in the queue have already retried recently)?

Am I doing something silly? Why are messages bouncing after two days when
the retry configuration indicates they should be tried for at least 4?

TIA,

Tim