[acdha]

Sandbox mode is also great for low-volume internal mail like notifications from cron.

My guess is that the main argument for sandbox mode is that it requires you to explicitly say you don’t spam and will handle reports, giving them carte blanche to yank your account if you do. Given how slimy email marketing is I suspect someone at AWS didn’t want to deal with people lying to their support people all of the time.

[throwaway823882]

And once you are out of sandbox mode, it scales pretty well. We regularly send millions of mails in 24hr periods. Some gotchas come in when you need to set up DKIM/etc and other providers may need to handle mail for the same domain.