I would recommend separating the service between transactional and marketing. Otherwise, there is always something going on with opt-out.

So use Sendgrid or Mailgun for transactional. Hit their API to send a mail. There are pros/cons around keeping the templates in there, but I would lean that way.

For drip campaigns, I would sync the product database to a tool like Mailchimp or the others ones that have been mentioned. We made an open source way to do that syncing, called Grouparoo.