First of all, we have a large software system we've developed and have a number of clients using it in their own environment. Each of them is responsible for using their own equipment and resources, we don't provide any services to share with them.

We have introduced an automated email system which sends emails automatically via SMTP. Usually, it only sends around 10-20 emails a day, but it's very possible to send bulk email up to thousands of people in a single day. This of course requires a big haul of work, which isn't necessarily the problem.

The issue arises when it comes to the SMTP server we're using. An email server is issued a number of relays a day, which is paid for. This isn't really necessarily the issue either. The risk is getting the email server blacklisted. It's inevitable, and we need to carefully take all this into consideration.

As far as I can see, the ideal setup would be to have at least 50 IP addresses on multiple servers, each of which hosts its own SMTP server. When sending bulk email, it will divide them up across these servers, and each one will process its own queue. If one of those IP's gets blacklisted, it will be decommissioned and a new IP will replace it.

Is there a better way that doesn't require us to invest in a large handful of servers? Perhaps a third party service which is meant exactly for this?