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This would not solve the issue when the email is sent to a mailing list.

When the mailing lists are internal Exchange-based lists Outlook can check how many people are on them.

Outlook does that by default when the email goes to more than a few hundreds people. Don't remember the exact limit.

It also works if it's a single destination email that happens to be a distribution list with 500 people.

They do it by default for all distribution list sizes, I have a list with 4 ppl in it and it still lets you know how many ppl will be getting your email.

IME email storms are most often for one of two reasons:

- someone mistakenly sends a mail to List B instead of List A, so everyone on List B goes "Huh? Remove me from this"

- someone correctly sends an email to List A, but List A was poorly configured and people were put on it without their knowledge and respond in confusion

A "too many recipients" warning wouldn't help with either of these cases.