Used Hotmail & Yahoo! Accounts
Spammers are very clever people. They particularly like to use the anti-spam vendors' tactics against them.
Here is a report of a Trojan virus called Trojan.Spammer.HotLan.A that leverages the disposable email address feature of Yahoo! and Hotmail to send out thousands of spam from legitimate email accounts.
Note that the classic techniques of bayesian poisoning (using literature to confuse the filters) and a random subject line affect the 'bulk-ness' algorithms by solutions like Commtouch's Recurrent Pattern Detection service.
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