The idea/term of "Outbox" really reminds me of email of the old days (in the 90s) where you have a desktop email client (e.g. Eudora, or Internet Mail and News, or even earlier versions of Outlook Express) that don't do auto-sends, and only connect to your POP3/SMTP servers when you click Send and Receive. This was probably due to internet being on dial-up and being billed by the minutes was common so users limited their internet time (e.g. disconnect from internet if writing a long email response, etc.). I have a lot of nostalgia from those days vs the always-on culture now.
I like that you were able to spot this particular usage pattern as something to narrow in on for your product. I don't know if this would catch on as a trend (for example, like ephemeral messaging did when Snap came out), but I wish you all the best!