Funny you mention AOL. There used to be a magic formula for how many concurrent SMTP connections, how many emails/minute, etc, you could pump at them before you silently went into spam.

I worked on a team with a large (real double opt in, not spam) email list. We had to pay for a bunch of AOL accounts to figure out the trigger points, so that we could get our email updates out overnight without tripping AOL's spammer logic.

And there was a time when they had dominant email share like Google has now.