I don't know if isps are in general allowed to refuse to send an email or refuse to allow a user to receive an email but I know that if I mail an MP3 using Gmail the chances are that Google meaning alphabet will refuse to transmit it based on

If I remember correctly

The digital millennium copyright act as currently amended

And in general

The principal that I shouldn't share a copy of a CD I own even track by track with someone else

However

I am pretty sure and this is a separate issue

That if I use an editor

And I've never tried

To put two MP3s together to make my own creation

Analogous to a well recognized principle that making your own CD mix is fine and you can share copies of that as long as it's not all the tracks from one album or CD and it represents some kind of authorship where you pick and choose music that meets your whatever needs or style or whatever

That probably would sell through but I don't have any hard data

I have had emails can that means canceled

Based solely on violation of digital millennium copyright act

And I note here there is no means for an appeal

So as far as net neutrality

An ISP who is not part of the backbone

Might be a good thing to have

An email from a secondary provider

Might be a good thing to have

Your mileage may vary

And that's just an opinion there about the backbone and the providers