Many in the thread have said what I would say: Don't mix your email with the sending of transactional emails. If you're comfortable with Heroku I recommend stay

Many in the thread have said what I would say: Don't mix your email with the sending of transactional emails. If you're comfortable with Heroku I recommend staying there and using something like SES, SendGrid, or Mail Gun to send the account registration emails. But, also remember, for most transactional email providers you could allow it to send as your employees if that is an important part of your needs. That way you can keep them separate and keep the ability to send email as your employees.