Job Firing?

Yes they can

Disruptive e-mails are whatever the company decides they are

Yes, they can and disruptive emails is whatever they believe constitutes disruptive emails.

Most employment agreements are "at will" which means the employer or the employee can terminate the employment at any time for any reason, as long as it's not unlawful.

"Disruptive emails" can include a whole lot. I think you probably know which emails got you in trouble.

You can be fired for any or no reason. Sending too many emails, sending non-business related ones over the business system, or sending ones containing abusive language would be considered disruptive.

Yes, definitely.

Of course they can. If you are sending messages or talking or even acting in a way that damages the interest of the company, they don't need to keep you - they didn't marry you or put you in prison, they just gave you a job.

An employer 'can' fire you for any reason. You are of course able to challenge the firing through legal or union ( if applicable ) means if you wish.