This is really interesting! Currently trying to solve a similar problem thing myself, so having a look through to learn from your solution.

I'd recommend having a look at AWS CDK to manage the AWS resource creation, is much nicer than raw CloudFormation: https://github.com/aws/aws-cdk/

For my solution to this I've also then got a frontend ontop of my backend to setup view mailboxes, email list per mailbox, email viewer (read/unread status), and email sending. I'm doing the same with S3/SES/Lambda + DynamoDB for email sending/recieving/processing + email metadata in DynamoDB. Mine is currently WIP and needs some real auth (using a URL API key, not the most secure...) + email replying + tidying up. Will definitely open source my solution and post to HN when ready.