I work with some non-profit organisations that use MailChimp for their mailings.

My employer sends out a few million mails per day and uses PowerMTA to process this workload.

The more mails you send out, the more you have to worry about email reputation. Major ISPs will throttle the throughput of emails from a sender with poor or unknown reputation, and allow full speed processing of senders with good reputation.

The main ways to build reputation are a) repeatedly send from the same IP address; b) Do not repeat sending mails to non-existant addresses; c) If your business has the money, work with an established organisation in this field to accellerate building your mail reputation.

The second part part of reputation building means you need to deal with bounced emails. You need to analyse the bounce records and mark bad email addresses so you do not send to them again. Most ISPs will tolerate a small number of repeats to bad addresses, but the more you send the lower your reputation will go.