By disclosing your e-mail address
This is the main reason why you are getting all this spam email. 95% of all junk mail you get is because you gave your email address to somebody, somehow, somewhere.
Ask yourself to whom you gave your email address.
Examples to whom you disclosed your email address
Sign-ups software downloads
From IRC and chat rooms
your business card
White & yellow pages
This list only contains the main sources where you disclose your email address.
Other sources of Spam Email
Through Mail Relaying
Mail Relaying is sending mail to a user that is not local to the server you have connected to. It slows down your email and gives spammers free bandwidth.
Through dictionary attacks
This happens when spammers connect to your server and just trying random address or even stepping through the alphabet. Some servers will block the sending server after X number of failures, some set max recipients per message and others set a delay between recipients to slow down the spammer, causing them to go elsewhere.
Through the selling of e-mail addresses
Companies sell your information to other companies.
By using HTML e-mails, spammers can confirm your email address. Best to disable preview window and delete the message. You can disable HTML or even block the email client from communicating on port 80.