There's absolutely nothing stopping my email provider (or yours) from blocking, editing or deleting my emails other than the terms of service we both agreed to when I started using the service.
Now, just like with the social media companies, the real governor is business. They wouldn't have many email customers if they behaved that way.
Also, don't be so sure your cell phone provider can't do the same thing - they aren't regulated the way AT&T was.
your examples merely illustrate my point the googleyoutubeFBtwitter conglomerate can do it because thy are more powerful than my cell carrier. They can eliminate political speech online, a significant amount of it. They can control the election by controlling what many see.