On 17 Jun 2020, at 11:07, Roberto Ragusa <m...@robertoragusa.it> wrote: > but when I start contacting them they easily complain with "too many > concurrent connections" because all the mx hosts have been resolved to the > same IP (well, IP pool, actually). These domains (not under my control) are > hosted on a provider that has shared SMTP servers for all their customers and > I do not even have an easy way to enumerate the domains.
This sounds like a badly configured host, and I imagine they have to do a lot of work to allow large mailers (like google or outlook) to bypass there configuration in order for their customers to receive mail. They are probably setting a limit of a single connection for any server they have not excluded specifically. They know they are elating mail by doing this, they just don’t care. I would try contacting them and if that fails (likely), add a footer to emails with something like “if your hosting provider is acme hosting LLC, your mail is probably severely delayed due to their misconfiguration of their server” and a link to a page explaining the issue. Since complaints from paying customers will certainly have weight if they ignore your query, this is much more likely to end up with a resolution. Or you are sending a LOT of mail to them (but I bet if you test, they are limiting to a single connection). I’ve run into this behavior in the past when I hosted several mailman lists, the only solution was to acknowledge that some users would have their list mail delayed by hours (or sometimes a day or two) and when they complained show them why.
