They can filter out the boxname part of [email protected], but they can't, in general, filter out [email protected] - it would break too many things. I guess it's possible to recognize mail for mytld.com is handled by FastMail, but I'm not sure anyone bothers. In my case, almost all spam I get comes to an alias I have in my Facebook profile, and the rest of it to an alias I put on my website - so in both cases, I assume spammers just scrapped the e-mail address.