Spammers can verify your address is active if you use the preview pane because they can send HTML-formatted messages. HTML is the same code that is used to create Web pages, so spammers can put a unique image in each message that will be logged whenever it is displayed. The code could be as simple as xxx.gif?a=youraddress@someplace.com.

Even if you don't open the message, if there is a tracking image in the first line of the message, it will be displayed in your preview pane and the spammer will log your address and add it to his list of active addresses for future bombardment.

Since Mailwasher reads the mail at Sympatico, I was under the impression that if it was deleted at source, the code wouldn't be executed.
Sorry!