I understand that for some messages, sure, you want an acknowledgement. I just don't see how that process is notably different for an async web client compared to what's already out there on desks and phones and particularly as compared to a blocking web UI.
If you had a blocking UI, any of those 'interruption' events could occur while you're staring at a spinner. And you (rightly) wouldn't know or feel confident that the message was sent until you re-established your connection and verified the item had made it to your sent items.
Which is the same as it would be with an async client: it's an important email, so until you saw it in the Sent Items folder, you wouldn't have the warm-and-fuzzy feeling.