For that kind of volume and the fact that it's basically "set it and forget it" to a single recipient, I'd be tempted to slap a Postfix server into an auto-scaling group, configure the rules internally, validate with your AWS rep that you're not going to smack face-first into their port 25 limits, and call it a day.

I hate running mailservers, but this is such a simple use case that I'd have a hard time dealing with the complexity of a new vendor relationship for such a direct use case.