Yahoo! and AOL changed their email policies which is affecting you. Please see this April 11, 2014 post from the Yahoo Mail Team and this April 22, 2014 post from the AOL Postmaster. Regarding this issue, you can see this posting at Mail Chimp who specializes in sending out emails to groups.

In effect Yahoo! and AOL now do not allow you to use your or address when you email all of your golfers/members. They consider that spamming or spoofing. You will have to use an email address in the "From" box that ends in something other than or One alternative would be to sign up for a free Gmail account and use your new address in the "From" box. If you have another email address you could use in the "From" field, please try that rather than your or email address. Unfortunately we have no control over policies such as this.

You can create an email group in your Yahoo! or AOL account and use that to send emails to your golfers. If you need help doing so, you will need to consult Yahoo! or AOL.