Well, it's possible that he's mildly delusional, as most of us are about beliefs about ourselves that we hold dear.
It strikes me that Ayyadurai is in a legal catch-22 situation. Let's suppose for a moment he did "invent" email. That would make him a public figure, and the legal standard used to establish defamation is "actual malice. That's a difficult standard to meet.
I assume Ayyadurai's complaint are claims that he is a "fake" or a "liar". Suppose some random shmoe is interviewing for a job, and you tell the interviewer that he's a "liar". That is defamation, unless you have actual reason to believe he is a liar. But if you say the same thing about a politician running for office, it's NOT defamation unless you have actual reason to believe he is NOT a liar. That's because the politician is a public figure.
It seems to me nearly impossible to defame someone by calling him a liar in the context of his claiming to invent anything. His very demand to be recognized for his achievement makes him a public figure, whether that claim is true or not.