[enhancement]: Allow the configuration of unique throttles per server

[BrunoBlanes]

Which feature or improvement would you like to request?

I run three separate Stalwart SMTP Server instances, each with its own IP space. Each instance can send up to 100 emails per hour to each recipient domain. I use this throttling method to reduce the risk of getting blacklisted by other systems.

Since most of these lists are either domain or IP based, I figured I'd be able to add those limits, but instead it is shared across all instances.

I would like to have the option of setting different rate limits for each server. This way, I could use multiple instances running in parallel, each with its own IP space, more efficiently. Moreover, it would be helpful to have a feature that monitors the queue and allows the available servers to deliver the messages that are waiting due to rate limits on other servers.

Is your feature request related to a problem?

No.

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