This article explains why an account may be temporarily blocked from sending email messages.  It also describes the next step the customer needs to take to get it unblocked.

To protect our users and our service from possible abuse, Yahoo! Mail can temporarily suspend a user from sending messages if we detect suspicious activity in an account. During the suspension, you can’t send email, but can receive messages and access to email in your Inbox and other personal folders.

If your account has been suspended from sending messages, we recommend you wait a few minutes before resending your message. We also encourage you to consider the following items to help prevent your account from getting this error in the future:

• Refrain from sending a message to a large number of recipients. If you need to send a single message to a large group of recipients, try sending it in small batches. (If you frequently send to the same, large group of people, we recommend you try Yahoo! Groups for this purpose.)
• If you need to send many messages, try not to send them within a short period of time.
• Check the contents of your message for something our systems might consider is spam.
• Finally, if you notice anything unusual that might suggest to you that someone else might be using your account to send messages, we encourage you to change your password on the Account Info page as soon as you can. You’ll be asked to sign in before proceeding.

If you still can’t send any email 48 hours after receiving the error message, please click Contact Yahoo! on this page for further assistance.