If you're using a mailto link in your contact form, you've done your client a huge disservice. That's just not how it's done anymore.
#1 mailto links require the end user to have a dedicated email client installed on their device. Something like Windows Mail or Outlook. Most people don't use email clients anymore. They use web mail services like gmail, yahoo and others so they cannot use your form. The prompt you're seeing is no doubt related to that. It's asking you to create an email client account on your system.
#2 Spambots & email harvesters are going to have a field day. There is nothing to prevent them from exploiting the form and gathering your client's email address to send even more spam. Before long, that inbox will be stuffed with so much junk mail, it will have to be shut down permanently.
Disable that mailto form immediately.
Contact the web host and find out which server-side processing scripts you can use on the hosting plan. Some hosts provide basic form-to-email scripts you can use. Others do not. You will need to script one of your own or find one from a 3rd party.
If your server supports PHP mail () function, this tutorial will take you through the whole process.
Alt-Web Design & Publishing: Responsive Contact Form with Bootstrap 3.2 and PHP (Part 1)
Nancy
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