My personal field of expertise is email marketing: template design and coding, deliverablity, data segmentation, etc.
I have experience with many ESP templating systems and features such as: Mailchimp, Aweber, Constant Contact, Campaign Monitor, Inbox25 and also WHMCS.
When designing templates, I always make sure that they render perfectly in all email clients using a service called [login to view URL]. I also always code my templates in a way that it is readable even with images disabled (as it is the case by default with most email providers: hotmail, yahoo, aol, gmail, outlook etc.)