This isn't how most client side email apps work. For example, on both the Mail app for mac and ios, the UI for sending is dismissed immediately. You know when it's sent when you hear the whoosh sound (or when you look to see that there is nothing left in your outbox).
Bottom line: you can have a non-blocking UI while still communicating to the user when there are problems.
Bottom line: you can have a non-blocking UI while still communicating to the user when there are problems.