It has happened several times to me that I hit reply instead of reply-all on an e-mail sent to several people. This caused problems in my work.

It would be great if there was a clear visible hint when replying only to the sender when the e-mail was sent to several addresses, or even a visible button [add all recipients] to fix the solution.

Maybe even swapping the reply and reply all buttons in the UI is enough, because it seems I often click on the first one. I find the icons not easy to distinguish.