registering requires email address
When you register a product you have bought or when you join a membership that requires you to enter an email address, your email address will get hit by advertisement email.
Create an email address for the registering products and other times when you have to give your email address but do not want mail. You will still get junk mail but it will be all under the email address that you don't use. Your regular mail will be clear of it. Be sure to periodically go in and delete the junk mail because most services will delete the email address if it is not used in awhile.
If you post on boards and want to leave your email address on the board, create an email address just for the boards. You then create a rule for email from the board to go to a specific folder. Then when you check the mail, you just check that folder. The rest should be junk mail and can be ignored.