The alternative is draft-first: the AI writes, a person sends. Auto-send is typically switched on per kind of email rather than for the whole inbox, for example order status questions but not refunds or complaints.
Why it matters
Auto-send trades control for speed. A reply that goes out in a minute is good service, but a wrong answer sent automatically reaches the customer before anyone sees it. The sensible order is to measure first: when drafts for a certain kind of email are almost always sent without changes, that kind of email is a candidate for auto-send. When they are often edited, it is not.
Questions to ask before turning it on: which emails does it cover, can you see everything it sent, and how quickly can you switch it off?
An example
A shop notices that replies to “where is my order?” are sent unchanged week after week. It lets those go out automatically and keeps drafts for everything that mentions money or a complaint.
In ReplyPen: Auto-send is planned for a later version, off until you switch it on. Today every reply is a Draft a person sends. See draft or auto-send.