I am an IT professional. In my experience this strongly depends on the browser you use. There are a lot of sites, that are capable of spying out your email. Of course just to for selling it to third party os use it for own spam purpose.
Just use a professional firewall which blocks your email literal string (after a warning) and you will see when your browser trys to send this information.
A good strategy is to keep browser and mail programs apart, which is definitely not possible with IE. But even this measurement I think does not beat every attack from hostile advertisement inserts in some web page. For instance ebay gives your email to "friend" sites like paypal just on the fly.