A few years ago, Thunderbird I think were the first ESP to check for the word “attachment” when you went to send and email, and if there was no attachment, it would say – “hey, we noticed you wrote attachment, but there is nothing attached.”
This was a fantastic solution to a common problem of people accidentally forgetting to attach a file that they were referring to in an email.
Today, I came across something working on the same principle by gmail. I referenced a doc on google drive in the email, and when I hit send, google informed me that not all recipients had access to the file.
It really became great UX when the pop up didn’t force me to cancel the sending of the email, and go dig around in google drive to find out who had permission to access the file or not, but instead asked if I wanted to grant everyone on the email list access to the file on google drive.