Do we need to change the PHP mailer script to cope with this or do you have a fix.
You should never send email on behalf of an @yahoo.com email address. Yahoo has made this crystal clear.
You should always send from a domain you have permission to send from.
To send mail on behalf of an @yahoo.com email address you will need to use your own domain in the "from" field, and use the @yahoo address in the 'reply-to' field.
In my experience, attempting to send from an @yahoo.com email address without having your message originate from an @yahoo.com mail server will ensure your message will be blocked.
Regards
Roger