Once that happens, the message moves to your "Sent" folder.
This has 2 big advantages.
1. You can see if/when your message was opened.
2. You can edit it as long as it sits in the outbox.

The only disadvantage is that you can get used to it working this way and then forget that regular emails don't! I can't tell you how many times after sending an email lately I want to edit it and then remember I can't!