RatelimitOutbound

Version one

To rate-limit outbound messages requires an assemblage of facilities. You will need to add a router:

• Early in the chain
• not active for verify
• limited to the items of interest
• redirect
• redirect supplied by an acl expansion (4.82 and later), calling an ACL which
  • tests using a ratelimit ACL condition
  • returns either the original recipient, or :defer:

You will also need to be starting queue-runners at some reasonable frequency.

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Putting the above requirements for the config together, an example:
ACL

#args  1:recipient  2:destination-class  3:rate-count  4:rate-time
check_for_rate:
 deny   ratelimit = $acl_arg3 / $acl_arg4 / per_cmd / outbound_$acl_arg2
        message = :defer:
 accept message = $acl_arg1

Router

ratelim:
 driver =  redirect
 verify =  false
 domains = awkwarddomain.com
 data =    ${acl {check_for_rate} {$local_part@$domain}{$domain}{15}{1h}}

Version two

The above isn't good if your preferred queue-runners use -qq mode; you get double-counting, and there's no programmatic visibility of the queue-run phases with which to fix it.

So: split the use of ratelimit into a read-only test, and an update.

• Do the test from the router as above
• Do the update from a msg:delivery Event
  • If your Exim version is too old for Events (pre 4.87), use a dummy expansion in the transport instead

Version three

Resource-constrained sites may feel that the processing done by the queue-runners hitting the ratelimit test is excessive. If there are only a small number of classes of limit needed, consider diverting the mails to an alternate named queue (4.88 and later), one per class. Instead of a queue-runner, for each holding queue run a cronjob script which moves a defined-size batch of mails back to the main queue for delivery. Just move the files (there should be two per mail) from one directory to another.