Recently (as of 2010) hotmail, yahoo, and gmail accounts have been under increasing attack by spammers because once they break into valid accounts, spammers can get around most spam filters (because email from known hotmail, yahoo, & gmail accounts are "trusted" by other systems and the email is sent to addresses from those accounts' address books).
The only current solution to this problem is for everyone to practice safe computing:
1) use very strong passwords on all of your accounts
2) don't use the same password on different accounts, use a unique one for each (so that if one system is broken into it does not expose a password used elsewhere)
3) be very careful when on unprotected networks or computers
4) keep your own computers and networks well secured and free from malware.
These are not necessarily easy things to do although tools such as LastPass (http://lastpass.com/) can make #1 & #2 easy (otherwise #2 is impossible for most people). #3 can be achieved but not using such or by being extremely careful. #4 can be achieved by being careful and only using programs on your computer from trusted sources. Additionally virus & spyware scanners may help, especially on MS Windows.