Microsoft had marked an email from a professor from an vt.edu domain email address as spam causing me to miss an interview for a PhD funding.
If one wants such legal protections, there is the post.
(Now, should there be such a right? That's an interesting question. But a world in which one exists would raise the bar to starting one's own email server even higher).
Negligent interference.
It's real unlikely any such guarantees were made. To do so would be extremely foolish for several reasons (the false-positive rate of spam identification is known and emails can fail to deliver because of an error at either end of the transaction).
If you want guaranteed delivery with proof and tracability, send a registered letter at the post office, FedEx, etc.