Mailto is a special type of HTML link. When the client opens this links, it will open the default mail client you have in your computer. The web browser requires a default e-mail client software installed on his computer in order to activate the e-mail client.
The mailto link must have the mailto:
prefix. This prefix will tell the browser that the client is trying to send and email and there for, open the email client. Followed by the prefix, it is required to input the email of the recivient.