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 t…
You can remove the "probably", it was exactly for that. Most often then not you would write offline, send the email (which would go in the "outbox") and then later on you would eventually connect to the Internet and send/receive emails.