Background information: We are a provider of web-hosting and email (among other services) and we need several load balanced SMTP servers to fix all our customers needs.
The problem: Every once and a while customers will try and send out an email with 500-1000 recipients and we have restrictions on this in our SPAM filter to prevent our smtp servers from being blacklisted.
What I need to know: Is there some way to balance the number of recipients between 3 smtp servers with seperate IPs to prevent blacklisting. Example: a customer adresses 900 recipients, 300 go to server A, 300 go to server B, 300 go to server C, this way they are much less likely to get blacklisted. What is the best option for a mail server for this and how would this sort of load balancing work? I am open to any MTA and any load balancing solution.