So here are two options I see for MX + SMTP on aws: https://docs.aws.amazon.com/workmail/latest/adminguide/howto-start.html 4$ per user per month managed aws service.
https://speedkills.io/email-server-aws/ ec2 instance install postfix + letsencrypt + dovecot + roundcube and other things to get a ubuntu system to handle MTA and SMTP with a basic webmail client with it.
Here’s a marketplace item for the preconfigured postfix setup: https://aws.amazon.com/marketplace/pp/prodview-7lthtvl7jhclu#pdp-pricing