PMTA config settings


Active Member
We have a bit of a problem with sending through PMTA. We use an external supplier for delivery server capacity and we will never run it ourselves since it's not our core competency.

Whatever we send, we never get more than 100 concurrent SMTP-connections on the delivery server end. The supplier said that they have configured their end of this to 10000 connections using this config setting

<source-group sourcegroup>
max-smtp-in 10000
reserved-smtp-in 50

We have partially developed our own virtual delivery server that funnels all our outgoing emails through that virtual delivery server whose only task is to pump the other side with messages using concurrent connections. This is handled with RedisQ and is blazing fast.

We can also configure the number of SMTP connections that we want to use. Each SMTP-connection is reused so it's super efficient (I wish that any of my devs described this instead of me :) ). When we are at 100 concurrent SMTP-connections in our virtual delivery server, we are at a constant 100% success rate, but when we increase just a little bit on our end the success rate drops, meaning, there it seem to be a limited number of connections on the other side (as we see it) and it seems that there are NO 10000 connections on the other end, despite the config setting.

Anyone who knows how to solve this? Are there other places for configuring this?

Thank you!


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*related above

This is the error that we got with more than 100 smtp conns on our end and delivery success rate drops.

Expected response code 220 but got code "421", with message "421 [] out of connection slots " (code: 421)