Add Content Editor Web Part on the page and in source editor insert the following code
<a href="mailto:abc@xyz.com">Email Us</a>
or 
open a site page in SharePoint Designer, select the location where you have to put the link and then insert above code
or 
if you want users to directly open email link on a page when they click on "Email Us / Contact Us" button then assign "OnClick" event to Contact Us button / tab with above code 
<INPUT TYPE="button" VALUE="Email Us" onClick="parent.location='mailto:abc@xyz.com'">