Emails sent to Yahoo go through a spam filter, if the subject line, senders email and content of the email look like spam the email will be redirected to your spam folder.
To ensure an email in not sent to your spam folder again please follow the steps below:
1. Go to the spam folder and open the email you wish to keep.
2. Click on the 'Not spam' button and the email will be sent to your inbox.
3. This means that by doing the above any future emails received from the email address will be delivered to your inbox.