[Mailman-Users] Mailman + Postfix - Too Many Connections

This is not strictly a mailman issue, but I would assume some people here would have encountered this issue.

We sent out mailing lists to schools here in Australia, and recently we've been getting "Too Many Connections #421" errors from them. Mails destined to gmail or hotmail are OK.

We're not trying to spam them (we have put links to unsubscribe, etc) but since the same mail server hosts emails for almost all the school domains, it's not liking us. I've set postfix variable of default_destination_recipient_limit to 15, still not liking it.

Any suggestions on rate-limiting on Postfix / mailman??

Please consider the environment before you print