I have a Postfix 2.5 server configured with a _destination_rate_delay of 12s which should limit deliveries to a particular destination to 5 per minute. On my reading of the documentation, a "destination" is the domain name of a recipient of a message.
My problem is that we are sending emails to three different domains, all of whose MX records point to the same mail server (the domains are for three UK universities), so while I may be able to limit deliveries to each domain, a total of 15 messages per minute could be sent to the remote server, exceeding its limit.
Clearly I could set the _destination_rate_delay to 36s, but I suspect that overall this would result in slower delivery. Is it possible to limit delivery rates by destination server rather than email address or is my reading of the documents incorrect? If not, can anyone suggest an alternative approach?