A lot of the free emails have started to crack down things. Generally, they limit how many you can send at a, this is to prevent spam abuse. Yahoo will not tell you what the limit is, "In order to better enforce anti-spam policies, Yahoo does not disclose the number of recipients or emails that can be sent at one time." But if pay them you are still limited, "Outbound email messages are limited to 500 per day. Each message can be sent to, at most, 100 recipients. Please note that Yahoo Small Business doesn't provide bulk email services. We recommend signing up with a third-party bulk email provider to create a mailing list."
I'd second the suggestion to use Constant Contact or Mail Chimp - both are better designed for large group emails and protect privacy. I personally get really annoyed when I get a bulk email that doesn't mask my email. That almost always leads to me getting on some junk mail list.