[exim] per recipient domain throttling

How can I throttle the number of delivery attempts to a given
recipient domain (NOT per MX)? Say, no more than one delivery attempt
every 5 minutes.

One of my users has her mail forwarded to gmail, and since gmail
(correctly) recognizes that lots of it is spam, it rate-limits me in
my attempts to send it - and then since all of it gets re-tried every
time the queue is run, it takes hours or days to get back to being
able to send mail to gmail without delay.

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