Question: When creating links for an an html based email, how critical is it to use absolute links? Also, What are the benefits in using relative links in general?

Answer: 
Email? I don't think there's an option other than absolute links. This will guarantee you that no matter who and how will read your email, your links will point to the place you need.
absolute links always start from the root website

```
/portal/images/myImage.png

```

relative links always start from the position of that page

```
../images/myImage.png

```

it is all about you when using this approaches, because I like to copy/past the hole web applications from server to server and sometimes place it in other sub folders, I tend to use Relative links more than absolute.

and in one domain root I always have several subfolders and some default files like:

* /webSite
* /portal
* /mail
* /iphone
* default.aspx
* robots.txt
* sitemap.xml

and inside everyone I have 

* /css
* /js
* /images
* files.aspx

instead of having a css in the root folder that contains all CSS files for all the sub applications for example, I always prefer to have all in just one place, it is better for me to move it somewhere else.

if you tend to build a web app to be permanent in that place, there is no warm to use absolute paths, it is a matter of taste and not best practice.

**HTML in the email** 

This is the place that you need to have the complete path 
 <http://www.mydomin.com/images/myImage.png>

hope it helps :)