I believe that the fact that senders are trigger-happy and receivers aren't great at reading suggests that the root problem behind both symptoms is that both groups don't value the time of others very well, which would explain both sides of the problem - although I'm open to alternative explanations.
If I’m visibly on the phone or you get a busy signal, you know I’m tied up. If you fire off an email, you can’t know if yours is one of two or one of two-thousand emails I have to process.
If I’m overloaded, there are emails that I won’t get to, pretty nearly by the definition of overloaded. If I’m overloaded and you have no signal of that, it’s not likely to go well for our communications, but doesn’t (necessarily) mean that I devalue your time.
How about something like this: for every 25 unread emails in the recipient’s inbox, the sender needs to hold down the ‘Send’ button for an additional second
(Of course, you can't know the actual destination mailbox [nor its contents] at the time of sending the email [forwarders, mailing lists, not wanting to leak information, etc]).
Maybe the problem with email is that is it such an old installation, in lack of a better word. I feel like there’s a better - almost similar - format waiting right around the corner.
In particular, sending has to incur some sort of ‘cost’ in terms of time or effort. This [0] needs to be doubly conservative.