> Why can't the other party be conservative in what they send?

Because they don't want to be? Because they don't know that they should be? Because they don't know how? Because they think they are but they're mistaken? Because the specification is ambiguous? Any number of reasons.

But you can't control what they do. You can only control what you do.

If you want to send an email to them, you need to be compatible with their email server.