Settings → Features → "Check client emails before they go". With it on, every email or text to a client stops at a preview page first: quotes, invoices, deposit invoices, statements, quote follow-ups, payment reminders, booking confirmations and project reports.
The preview shows exactly what the client will get — the From address, who it's going to, the subject, the email as it will look (signature included) and any PDF attached, with its size. Nothing has been sent at that point. Send now sends it as shown and does everything a normal send does (marks the quote or invoice sent, logs it, syncs it to Xero/QuickBooks/Sage). Back to edit — don't send drops it and returns you to the record.
Buttons change to say so: Save & preview on the quote builder, Preview & email on quotes and invoices. Booking confirmations go by text first; the preview shows the text and the email that goes instead if the text can't be delivered.
Off (the default), everything sends the moment you click, as before.