Watch your free tier very closely. An unfortunate part of being in the email business is that spammers and phishers are always looking for free ways to send their emails on ips with a good reputation. It quickly becomes a game of cat and mouse which detracts from the business, so make sure you have a effective strategy to prevent misuse when allowing customizable emails with a generous free tier.
Thanks a lot for the advice here. That's definitely an area where we spend a lot of time and one of the reasons why we've been under a private beta for the last six months.
This point does need strongly reinforcing - that you have a free tier at all creates conditions for sharing an outbound prefix with low-reputation senders. Itβs a definite barrier to adoption, and one you can only lower with an engineering-based explanation of your mitigations.
