The code below is not a replacement to the well known workflow + plug-in solution. This is a simple service function that I usually add to the email entity onload event, so when a user clicks on the send email toolbar button the opener ( related entity ) link is embedded in the email body.
Users find this extremely comfortable since even if you don’t send the email to a co-worker as a direct reference when the email comes back it very handy to be able to open the record from within outlook like a bookmark.
if( crmForm.FormType == 1 && opener )
var opnrLoc = opener.location;
var relUrl = "http://" + opnrLoc.host + opnrLoc.pathname + opnrLoc.search;
var ifrmDoc = document.all.descriptionIFrame.contentWindow.document;
var relLink = iframeDoc.createElement( "A" );
relLink.href = url;
relLink.innerText = "Related Record";
ifrmDoc.body.appendChild( relLink );