If you are using a Yahoo email address as your website email address - i.e. the email address that your website sends out email from - this could be the reason your emails are not getting sent from your website to your members with Yahoo email addresses.

The easy fix is to create your own customized email address, i.e. support@your_website.com - or use a different email provider like gmail.

Upon troubleshooting this for a UltimateWB Full software/web hosting customer, we found that Yahoo sent out this error message for the emails not getting sent:

The mail wasn't delivered because Yahoo was unable to verify that it came from a legitimate email sender.

Your email failed one or more of the following checks that Yahoo uses to verify emails are truly sent from the domains they claim to originate from. 

The best option to prevent such issues is to change the "from address/sending email address" of the contact form in your website.

You can also get more info on this from the Yahoo webpage here: https://help.yahoo.com/kb/postmaster/SLN7253.html .

You can set up your customized emails using your UltimateWB website control panel.  Here are the tutorials on that: https://www.ultimatewb.com/web-hosting-cpanel-tutorials . And the direct link for "How To Create Your Own Custom Email Accounts": https://www.ultimatewb.com/web-hosting-cpanel-tutorials/how-to-create-your-own-custom-email-accounts .

Don't have UltimateWB web hosting?  You should be able to set it up using your web hosting control panel purchased from another company if they offer that service - just ask them.  You can purchase UltimateWB web hosting packages here: https://www.ultimatewb.com/domain-names-web-hosting

After choosing/creating a new email address for your website to send emails from, you will need to update this info on your website admin panel. You can do this on the Admin > COnfigure Site > Social Configurations page, "Email Settings" section.