“pop-up on send” is a safety net. It is not a technology which has been eclipsed. It should be an option which can be turned on or off by the user.
When in the middle of putting my thoughts down in an email, I do not want to stop and fix my misspelled words. At that moment, I want to put down my thoughts as best I can. I will spell check at a later time. There are times when we forgot and hit send. That is when the pop-up spell check reminds us that we have a problem.
I use Grammarly. As I type this post, it autocorrects things for me. I do not stop and make the corrections at that time. I do it in the end.
This leads to a new question. Is Grammarly available in eM Client?