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Today on the yahoo email account I use for tech support clients, I received the CAPTCHA image that comes up every now and then. I noticed that even after properly entering the captcha, my emails were not showing up as SENT in the SENT outbox. 
Then I tried to email another client, and got a message from Yahoo that all emails had been blocked for 24 hours because their system had detected SPAM activity. Of course, I never use this account for SPAM nor am I a SPAMmer. Matter of fact, I never even use this Yahoo email for anything other than response to clients who email me first.
I found this as well
Yahoo! Answers - I was blocked as suspected spam by yahoo after 3 e-mails. Can I talk live to anyone at yahoo?
similar situation.
Yahoo's SPAM filters are obviously too sensitive! Not to mention flat out wrong.
Then I tried to email another client, and got a message from Yahoo that all emails had been blocked for 24 hours because their system had detected SPAM activity. Of course, I never use this account for SPAM nor am I a SPAMmer. Matter of fact, I never even use this Yahoo email for anything other than response to clients who email me first.
I found this as well
Yahoo! Answers - I was blocked as suspected spam by yahoo after 3 e-mails. Can I talk live to anyone at yahoo?
similar situation.
Yahoo's SPAM filters are obviously too sensitive! Not to mention flat out wrong.