Cancel sending message (undo)

[chaserene]

Is your feature request related to a problem? Please describe.
it's way too easy to accidentally send a mail you're composing while you wanna tap something else in the top bar. it just occurred to me that I was trying to add an attachment, but my tap was imprecise and I instead sent the message. another time I was trying to quickly exit the editor, but due to the arrow-like shape of the send icon, I tapped that instead of the actual back arrow.

Describe the solution you'd like
an opt-in option to insert some type of confirmatory action between touching Send and actually sending. e.g.:

• require long-tapping Send for 2 seconds, progress could be shown with a circular bar around the icon
• display a swipe-to-send modal
• display a simple Yes/No modal

[Monkey-Matt]

What I'd prefer is an undo option similar to what we see in gmail. So basically it doesn't send the message directly when you press send but instead queues it up and shows a snackbar with an undo button, which if clicked stops the email being sent, or after x seconds goes away and then sends the message.
This has the advantage of not getting in the way of regular use but still allows you to recover from mistakes. I imagine this being an option you can turn on/off same a gmail.

[cketti]

I also prefer the undo approach.

The implementation is a bit tricky because the MessageCompose activity is closed when the send button is pressed. Gmail displays a snackbar with an undo action in the message list screen. I'd like to copy this approach.
However, it's also possible to open the compose screen from outside the app, e.g. via a mailto: link from the browser. Pressing the send button will return to the browser, not K-9 Mail's message list screen. In that case we should probably add a system notification that allows the user to undo sending the message.

The functionality should be client-side only, i.e. we don't actually hand over the message to the outgoing server until the "undo" timeout has expired.

[schmitmd]

Possibly related to #624

[mosiser]

Anything new?
I think this is a fundamental function, it's too easy to press the send button accidentally, and you cannot delete the sent message as with IM...

Personally, it's enough to not advert this software to my relatives/friends, and that's a pity...