Then route your messages into S3, mount the bucket using s3fs, and point Dovecot at it (Maildir) to enable IMAP. Or just use a MUA that could read a Maildir dir

Then route your messages into S3, mount the bucket using s3fs, and point Dovecot at it (Maildir) to enable IMAP. Or just use a MUA that could read a Maildir directly. Going the other way, you could configure Postfix to relay via SES.