I'm surprised this is the first time anyone has mentioned this in the thread. Throttling is incredibly important. As Joe Blow web app, you can't just connect to Google/Yahoo/etc and start cramming 10,000 emails a second down their throat. They'll drop the connection, and at some point, you won't be able to connect at all.

There's a size range between the little guy, who has no need for large, scalable email delivery, and the huge players who don't get shut out because of their name. For these people, it involves a lot of footwork. You have to identify where the bulk of your email is headed and establish relationships with those entities. This means talking to them and understanding their limits.

Or you simply outsource part of the job (e.g., Return Path) or the whole thing (e.g., Send Grid).