To make delayed/planned sending of emails possible
The Send Later extension presents a complicated dialog window that must be reviewed, configured & submitted for every outgoing message. This is far more complex (and annoying) than what we actually need:
What we really want is a toggle in preferences that (when enabled) puts all outgoing messages into the Outbox by default when you press the “Send” button in the message composition window, so outgoing mail is only transmitted when you fetch incoming mail with the Get Messages button. The functionality that I am describing here is (or was) a feature of Microsoft Outlook, and that is the feature from Outlook we miss the most. (The only reason we switched to Thunderbird is that we want the ability to migrate mail user profiles between OS platforms as needed.)
In our application, the purpose of not sending mail immediately is to facilitate group collaboration: when several people are working on a related project and you need to reply to them all, sometimes you need to add user-specific instructions to one or more recipients, and may not realize this until you have read all new messages for the day. If you have already transmitted a message, you cannot add something to it. When all of the outgoing mail from the current session is held in the Outbox, you can amend messages to individual group members as needed. Since we NEVER schedule messages to be sent at a particular time, the Send Later extension is a nuisance which adds unnecessary steps to our workflow and creates the potential for misconfiguration by users.
This is very important, these days one tries not to send business emails at weekends or out of hours. I tried the add on, it didn't work very well, a test email to me also sent a completely unrelated one from way back. Can't risk that happening.
The Send Later extension no longer works on Thunderbird 115. Not having this option is enough of a dealbreaker to force me to revert to version 114.
Just like to point out for those wanting basic send later function to place mail in the outbox the keystroke is Ctrl+shift+Enter.