Autoresponder
An autoresponder automatically sends an email, or a series of emails, to subscribers based on specific triggers or time intervals.
An autoresponder sends a prewritten email, or a short series, when a trigger occurs, such as a signup, a form submission or a download request. It is the simplest form of email automation: one trigger, one predefined reply, nobody pressing send.
Typical examples are subscription confirmations, form acknowledgments, download delivery and event registration confirmations. A welcome email is an autoresponder when it fires on signup. A multi-step sequence spaced over days is usually called a drip campaign, though the mechanism is the same.
Some autoresponders are also transactional emails, such as an order confirmation. Whether a message counts as transactional depends on its content and purpose, not on the fact that it was sent automatically.
Common mistakes
Writing one reply for everyone. A confirmation that does not say what the person signed up for, what arrives next, or when, gives them nothing to act on. Another is letting a trigger fire more than once, so someone who submits a form twice gets the same email twice.