Agreed, but there's no delivered (but not necessarily read) receipt that applies to email so that was closest I could think of that counted. The overall point remains: sending an email, with no further evidence, does not count as proof of delivery (all the way to the inbox).
I see what you mean, and lack of a rejection email from the server is not proof of delivery either.
I was looking to things like state process service laws. It doesn't seem like any type of receipt is required for electronic proof of service in California, for instance https://law.justia.com/codes/california/code-ccp/part-2/titl...