A transactional email is a specific type of email message that accomplishes some kind of individual transaction. An important distinction of transactional emails from other types of emails is that they are sent to one person, rather than to many people. Another distinction is that transactional emails include form tools for accomplishing some type of transactional task.
Techopedia explains Transactional Email
Examples of transactional emails include emails that allow for setting up an account, changing a password or registering for some type of product or service. Many different types of commercial businesses may send transactional emails, including retailers, e-commerce stores, service companies and even schools and universities.
Companies that send transactional emails may spend quite a bit of time looking at what works best and what recipients will respond to. Companies have developed sets of "best practices" for writing and creating transactional emails. For instance, transactional email senders respond to the ways that readers read these emails, often skimming or scanning them for information. They may also include particular features like a reply-to address, information about where the message is coming from, and appropriate subject lines to help orient readers.
Another important issue about transactional emails is how much plain text is in an email, as compared to how much functional HTML code is involved. The more functionality the transactional email is meant to facilitate, the more HTML would need to be included. For instance, some transactional emails for reservations would need a lot of specific hyperlinks and even some types of pointers to where recipients can enter information about themselves in contact fields.