Read receipts
A read receipt is a short message a recipient's mail app sends back when they open your message, if they agree to (RFC 8098). Many apps never send one, and nobody has to, so no receipt does not mean the message was not read.
Asking for one
When you write a message, tick Ask for a read receipt under the message. The receipt comes back to the address you send from: a receipt can only be asked for to one of your own addresses.
To ask on every message, open Settings → Writing and tick Ask for a read receipt on every message I write; you can still untick it on one message.
When someone asks you
A message that asks for a receipt shows The sender asks for a read receipt above it, with Send receipt and Don't send. Either way the message is marked, so you are not asked again, in webmail or a mail app that reads the same mark ($MDNSent).
In Settings → Writing, When a message asks for a read receipt chooses:
- Ask me (the default).
- Always send one: sent as soon as you open the message.
- Never send one: no question, and nothing is sent.
Your own messages, and messages in Sent, never ask. These two settings are kept in this browser.
Read receipts or open tracking?
Plans with read tracking also have Tell me when it's opened, which adds a tracking image instead and does not ask the recipient. See JMAP for apps sending receipts themselves.