The javascript code below creates an ActionButton which I detailed in a previous post. The code has been modified to add the emailing functionality. I've used the attribute emailaddress1(Email) for this example.

//replace 'emailaddress1' with the name of your custom attribute, make sure you keep the '_d' ending
function loadjscssfile(filename, filetype){  
  if (filetype=="js"){  
    var fileref=document.createElement('script')  
    fileref.setAttribute("type","text/javascript")  
    fileref.setAttribute("src", filename)  
  }  
  else if (filetype=="css"){  
    var fileref=document.createElement("link")  
    fileref.setAttribute("rel", "stylesheet")  
    fileref.setAttribute("type", "text/css")  
    fileref.setAttribute("href", filename)  
  }  
  if (typeof fileref!="undefined")  
    document.getElementsByTagName("head")[0].appendChild(fileref)  
}  
//load some styles to display link image and hovering link image  
loadjscssfile("http://" + location.host + "/ISV/styles/openEmail.css", "css")  
  
var linkTD = document.getElementById("emailaddress1_d");  
var siteIMG = document.createElement("img");  
var siteA = document.createElement("a");

//create img overlay to show "Send Email to Contact" when icon is hovered and store javascript to open link  
siteIMG.style.width = "22px";  
siteIMG.style.height = "19px";  
siteIMG.setAttribute("title", "Send Email to Contact");  
siteIMG.setAttribute("alt", "Send Email to Contact");  
siteIMG.setAttribute("src", "http://" + location.host + "/ISV/styles/blank.gif");  //1x1 pixel blank overlay image  
siteIMG.style.visible = false;  

//create img  
siteA.setAttribute("href", "mailto:"+crmForm.all.emailaddress1.DataValue);  
siteA.style.position = "absolute";  
siteA.className = "openEmail";  
siteA.style.right = "2px";  
siteA.style.top = "4px";  
siteA.appendChild(siteIMG);  

linkTD.style.position = "relative";  
linkTD.appendChild(siteA);

The images and files used in the sample can be found here: Download Zip

Make sure to put these files in "C:Program Files\Microsoft Dynamics CRM\CRMWeb\ISV\styles" the ISV folder of your CRM 4.0 on premise install. By default, a 'styles' folder does not exist so please create one.
While this feature might seem trivial, every little bit helps in ensuring a great CRM experience!