One of the most common issues of Yahoo Mail, the Yahoo error code 554, is an SMTP (Simple Mail Transfer Protocol) error that occurs when someone tries to send an email. If you see that Yahoo Mail not working because of this error, you can follow the below-mentioned steps:
1. While sending an email, ensure that the recipient’s email address is entered correctly and is active.
2. Ensure that you are not violating Yahoo’s Terms of Service and that the email content is properly reviewed.
3. Don’t use embedded objects, HTML forms, or JavaScript.
4. Ensure that the Yahoo SMTP server settings are configured correctly.