Ratelimit hit - Do emails get queued or rejected?

Cowsayswhat

We’re using a Smarthost with strict rate limits (very few emails/minute). When we exceed this limit, emails are blocked outright. they don’t queue, and our ticket system disconnects the inbox due to hard errors. This creates significant disruption.

Question: If we configure Mailcow to use this Smarthost and set Mailcow’s own rate limit below the Smarthost’s threshold, will Mailcow queue the excess emails internally to prevent hitting the Smarthost’s hard limit?

Specifically: When Mailcow’s rate limit is reached, does it:

Queue emails locally and retry sending to the Smarthost gradually, or
Reject them immediately (similar to the Smarthost’s behavior)?

Please don’t suggest using another provider - this decision isn’t up to us.

ETNyx

If you are talking about rate limiting provided by MC UI, than NO. This limit will end up in Postfix rejecting message over limit whit soft reject, so it’s MUA’s job to handle it’s queue.

On the other side as admin you can overcome this. Make MC UI limiting unlimited (or empirically decide about 2x/3x limit you usually need, maybe,..). And to ensure your next hop is not rejecting due to his rate limiting reconfigure your MC Postfix by those directives:

smtp_destination_concurrency_limit = 10
smtp_destination_rate_delay = 10s

Let’s say you submit 100 mails to gmail and 100 mail to outlook at the same time

0s:  10m -> gmail + 10m -> yahoo (180m in queue)
10s: 10m -> gmail + 10m -> yahoo (160m in queue)

Those limit can be done per destination, so let say 10 for gmail and 20 for yahoo. Or in your case just limit your next hop smart-host as you like,…