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1. Spammers often take a domain they know is legit and add on many common prefixes like firstname.lastname@example.org or email@example.com and sometimes any possible 3 to 4 letter combinations. submissions seems a bit less common but common enough.
2. Your email address has been sold by one of the search engines to a spam mail list company, and heaps of people have bought the list.
im sure they have lots of other things up their slimy sleeves.
I cant remember the name of the protocol myself (maybe some one can tell me), but your ISP can catch your email address, and when you go to a web site the sites server can request your address from your ISP if they have it enabled! Doubt this happend to you but there are lots of other thing that could go on.