Yahoo! Mail is a free Web-based email service managed by Yahoo!, Inc. Like most other email servers, the Yahoo! Mail server uses a blacklist that contains the Internet protocol (IP) addresses of senders of unsolicited messages. If the IP address of your mail server is on that blacklist, Yahoo! Mail automatically rejects emails sent from your account. Check if your mail server's IP address is on Yahoo! Mail's blacklist by following a specific procedure.
Open your email application.
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Click "New message" or "New." Type a few words in both the email's body and subject field. In the "To" field, type a Yahoo! Mail address. Yahoo! Mail addresses typically end with "@yahoo.com."
Click "Send." Wait at least 15 minutes for an error message. If your IP address has indeed been blacklisted by Yahoo!, the Yahoo! Mail server will automatically send you an error message telling you that your message cannot be delivered due to user complaints. If no message is sent by the server, your IP address has not been blacklisted.