4) Policy says that you will accept 10 messages an hour from someone. What do you do with the 11th message?
Don't accept it. The real answer, of course, is how you don't accept the message. There are two options: temporary refusal (4xx response) and permanent refusal (5xx response). In this case, the most appropriate thing to do is return a 4xx response to the message. You don't want to start bouncing messages because an MTA went down for an hour and has a small backlog for you.
Intelligent MTA design might have an escalating series of responses. For example, you could take the 11th through 1100th message and return 4xx responses, then start sending back permanent refusals (5xx responses) because it's clear that something is wrong on the other end that is not quickly getting better.
In any case, immediately responding with a permanent refusal (5xx) may be more emotionally satisfying, but is not good practice.
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