In my last post about qmail I said that once you solve the big concurrency problem you'll end up with another problem because your mail server would create too many outgoing connections to some domains and you risk having your ips banned by those servers.
The solution is to have a way of limiting the maximum concurrency rate by domains. To do that you'll need the qmail channels patch or write your own patch like I did ( mostly because I was unaware of the existence of the qmail channels patch )
The home page of the qmail channels patch will explain how to setup and configure qmail to limit the concurrency by a domain or group of domains.
What I like about this patch is that it allows you to set a concurrency limit for a group of domains like set 100 for all yahoo.com, yahoo.co.uk, yahoo.ca, etc .
What I don't like is that it doesn't seem to be able to set a default concurrency level for any domain. If I'm wrong please correct me, but if I'm right then this seems like a major problem for an email server that sends to a large number of addresses distributed over a large number of domains because you would have to configure concurrency limits for a lot of domains.
The ideal solution would allow you to specify a default per domain concurrency and this would apply to any domain that doesn't have a specific concurrency. For example most email servers would be ok with 5 concurrency connections from the same ip but no way for AOL (unless you're white listed and maybe not even then ) .
Another feature I would like is to be able to specify concurrency by domain's MX records or ips/group of ips assigned to the MX servers instead of the actual domain. This would ease the configuration for ISPs that host a lot of domains like rr or yahoo.