How do you normally read your email?
Do you use a mail program on your computer connecting with POP3 or IMAP, or do you use your web browser and the Yahoo Mail web page?
Regular logins to the Yahoo Mail site are required.
Can your contacts send you a new email? Does the email only bounce when they reply?
Can you copy and paste the exact error message they are receiving?
Try going to the settings and adding/verifying a phone number for your account.