If a message failed to send e.g because bad connection I would like to informed that this message is not sent.
And also I would like a option to resend it.
If a message failed to send I only see (very often by chance) a “x” instead a check mark beside the message.
UI-sided it could be a push message like for received messages but with two buttons like “retry to send” and “give up”.
See also my Screenshot to illustrate my idea.