receiving 50 to 75 emails a day wanting to sell me something

A lot of these emails are sent out initially at random: the people who send them don't know if your email address is a real one or not. When you click 'unsubscribe' you are telling them that it's a real address and that the user actually reads his/her mail -- so they send more mail to your address and sell your address to others who send even more. Moreover, when you post your email address on the internet (as you have just done above) you are telling the whole world that it's an address in use, so you are exposing yourself to even more. So a good rule is never to give out your address except to trusted correspondents and never to unsubscribe from 'offer' emails unless they are from reputable companies with which you already have dealings. There's not a lot you can do to prevent such emails from arriving, you can only hope to make them easier to delete. One way is to try to train Thunderbird to spot them (and, if you wish, move them automatically into a separate folder); another is to implement the 'spam filter' that may be offered by the company that provides your email service. But neither is likely to be 100% effective.