I have also got this error once in a while.
Yahoo customer service response is
"You can send the same message to up to
100 people at a time. To prevent spammers from taking advantage of this
option, no more than 50 of those addresses can be within the same
domain. For example, you can't send more than 50 messages at a time to
people who have addresses at yahoo.com or another email provider
address.
To send your message to more than one person, add additional email
addresses in the To, Cc, and/or Bcc fields separated by commas:
johndoe@yahoo.com,
janedoe@yahoo.com, etc.
You can also select a list of recipients from your Yahoo! Address Book,
so the email will be sent to all contacts on that list. "Everyone" is a
list that contains everyone in your Address Book. However, if you have
over 100 contacts, using "Everyone" or another list you've created, your
message will not be sent.
If you do not enter valid email addresses, or if you do not use commas
to properly separate the addresses, your message will not be sent.
Instead, you will see the message: "Message not sent. There is a syntax
error in your address field(s)..."
Remember that to send a message, you must always have at least one
address in the 'To' field. Also note that sending junk mail or "spam" is
a violation of the Yahoo! Mail Terms of Service.
If you want to send messages to a large number of recipients, create a
Yahoo! Group and use it to send emails to everyone in that group. Learn
how to create a Yahoo! Group at:
http://us.rd.yahoo.com/CC-us/mail/ Yahoo! Groups - Join or create groups, clubs, forums & communities
"
The message disappears gradually but I am not aware and will like to find out as to how long it remains in "blocked (human verification required)" state ?