Yahoo! Mail Send Limits: 100 recipients / message
Yahoo! Mail Send Rate: 500 emails / day
Yahoo! Mail Attachment Size Limit: 25MB
If you are sending messages to a significant number of Yahoo! Mail users, you may try to reconnect to Yahoo! Mail MX servers immediately thereafter.
Yahoo! Mail also keeps reputation data about each IP address and domain you send from. Therefore, by segmenting the mail stream that delivers marketing mail from another that sends transactional messages, you make it easier for Yahoo! Mail to determine and assign each mail stream’s reputation.
If you use Yahoo! Mail Plus, the outbound email messages are limited to 500 per day, and each message can be sent to no more than 100 contacts at once.
Further reading: Yahoo! Mail Basics
