Mail sending error not visible if tab exited during undo send period

Steps to reproduce

1. Open the app
2. Configure an account
3. Compose a new message to an invalid recipient
4. Send it

Expected behavior

1. Toast shows Sending message and the Undo button.
2. Message is added to outbox
3. I get an error toast that the message could not be sent
4. Message remains in outbox

Actual behavior

1. *Toast shows Message sent' and the Undo button
2. Message is added to outbox
3. I get an error toast that the message could not be sent
4. Message remains in outbox

^ I think this is especially bad if the user either closes the tab/browser thinking the message definitely went out, or when they are on another tab and don't notice the error toast.

Mail app version

1.3, 1.4

Mailserver or service

No response

Operating system

No response

PHP engine version

No response

Web server

No response

Database

No response

Additional info

Feeback from @marcoambrosini

Hello, today I ran into something with mail:

• I hit send message
• The toast confirmed that the message was sent
• I found the message in outbox a few hours later with the warning "Message could not be sent"

...meaning that the toast was lying!

I think the text of the toast should be changed into sending message in 3, 2, 1 or whatever but it shouldn't fool me into thinking that a message was sent when it wasn't 😅

cc @jancborchardt @nimishavijay

Reproduce the invalid-recipient send flow during the undo period and trace the mail send UI that displays the toast. Check how the toast changes when sending later fails and how the outbox state is presented. Done means the initial notification does not claim the message was sent before delivery succeeds, including when the tab is left open or closed.