Hardly something to worry about. Anything but messages written for and sent to one person is volume E-mail. That's with a cost per mailing of between 30 and 80 cents. But junk E-mail is very ineffective. Some reports suggest rates well under one response for 10,000 mailings.
Most people just discard it as quickly as they can.
Some go out of their way to cause trouble for the mailer. Put any real cost on internet junk mail and it goes away, because it is no longer cost effective. Even a cost of a penny per message dooms junk E-mail as a marketing medium. And one to one messages cost way more than a penny. One to one messages cost more than phone calls.