That is a website option of your e-mail provider. You must ask them for more information. Note: the receiver can override that setting, if they know how.

My email provider is Mozilla Thunderbird. I thought this is a forum for them.

Thunderbird is a browser for e-mail. Not a provider. Your provider is; YourUserName@Provider.com

Sorry, this forum supports the Firefox browsers as well as Thunderbird. An administrator or moderator can move your question.

A delivery status notification or DSN usually is a report from one or more servers on what they did with your message. For example, your sending mail server may report that it successfully forwarded (relayed) the message. Some servers send them and some do not.

This is different than a read receipt request, which triggers an action by the receiving mail software, assuming the recipient hasn't turned it off.

By the way, if you need to access this setting often, I noticed there is an add-on which adds a toolbar button: https://addons.mozilla.org/thunderbird/addon/return-receipt-toolbar-button/ (I haven't tried it myself)

=>moved thread from Firefox to Thunderbird section.

FredMcD said

Thunderbird is a <strike>browser</strike> for e-mail. Not a provider. Your provider is; YourUserName@Provider.com

Thunderbird is a email client.