Ask HN: How to send emails without hardcoding them in source code?

The large transactional email sending services (sendgrid, mailgun, postmark, ...) support email templates, e.g. https://sendgrid.com/docs/User_Guide/Transactional_Templates... and some have online editors. Then in your code you call an API method with placeholders. https://postmarkapp.com/developer/api/templates-api

Populate a template using a list of recipients, a subject, a body, which could be in a database or a file.

This is where I would start: http://bfy.tw/JMKG