I’ve ran postfix/dovecot for email in the past. Postfix would be the more important one as it’s the MTA. You’ll need at least one IP and a DNS registrar. That stuff isn’t so bad, the tricky part is ensuring your email doesn’t get sent straight to spam. I also configured dmarc, dkim, spf, etc to make sure my server was verifiable.

From what you’re saying; 100k emails a month from a new mail system is gonna get you on a spam list or blacklist. If no one reports you it might not be an issue. I don’t know if ISPs limit that stuff.

If you’re good with this stuff. You should be up and running in a day or so, but I’m curious to see how long the operation lasts. Most postfix how-to docs can get you started