For a campaign, I needed a 2px dotted line border around a section of content, and a horizontal dotted line inside. CSS dotted lines stink. These needed to be round dots. For all clients accepting of CSS background images, I just sliced out 3 vertical and 3 horizontal dots from my PSD and tiled them in the HTML. No sweat. However, for Outlook, this wasn't exactly working. I used the VML background hack and it worked horizontally, but not at all vertically. SO I started to dig into the Microsoft Dev page on VML elements. I ended up testing and using this snippet of code, that worked out almost great. The only hang up is you have to define a height, which isn't a deal breaker, but a bit annoying.

For the vertical line:

<!--[if gte mso 9]> <v:line style='position:relative;' from="0px,0px" to="0px,700px" strokecolor="#002663" strokeweight="2pt"> <v:stroke dashstyle="dot" /></v:line> <![endif]-->

For the horizontal:

<!--[if gte mso 9]> <v:line style='position:relative;' from="0px,0px" to="622px,0px" strokecolor="#002663" strokeweight="2pt"> <v:stroke dashstyle="dot" /></v:line> <![endif]--> <div></div><!--[if gte mso 9]> </v:textbox> </v:rect> <![endif]-->

I'm sure you get the idea. The "from" and "to" define where the line begins and ends. There are also a number of different dashstyles that you can use, defined on the MS Dev site, though I haven't tested them all.