So the value prop is markdown. Besides that, you have to compete equally with Mailchimp, etc which is impossible. Why not offer your markdown parsing as a service layer on top of existing providers' APIs?
If a new competitor enters this market that competitor will have to solve this problem again without being able to provide an additional benefit to customers.
So, building your own email delivery infrastructure - even if by merely using existing cloud-based infrastructure such as AWS SES - might prove to be an uphill battle.
It might still be worth it though in order to remain independent of third parties.
Why would they need to compete equally with Mailchimp?