There are a lot of questions about bulk emailing. I was trying to find one to close Linux Mail Server Architecture for bulk mailing as, but didn't find anything.. specifically canonical..
Here's an idea.
Q: My $employer wants to be able to send massive amounts of email, what's the best way to achieve this. A: Don't.
A: Discussion of hosted SMTP services, hosted mailing list processors (without making recommendations).
A: Discussion of how to keep server reputation high, DKIM, SPF, unsubscribe, CAN-SPAM.