Can I use transactional emails for marketing or promotion?

As an additional benefit – Yes! Exclusively for marketing – No!

Transactional emails have specific purpose – to communicate information to the customers based on their actions. Ideally, you must use marketing emails (email campaigns) for exclusively promoting your products or services. However, considering transactional emails have higher click-through rates, you might want to leverage the benefits. So, it is alright to use transactional emails for marketing along with their usual purpose – but of course, in moderation!