There's been a lot of hype around the potential of transactional e-mail to deliver marketing messages -- and it's well founded. Nothing is more potent than delivering a cross- and upsell message that's tightly tied and perfectly timed to consumer behavior.
With spam filters wreaking havoc for e-mail marketers, the convergence of marketing and transactional e-mails isn't surprising. Unlike bulk marketing e-mail, transactional messages stand a better chance of reaching the inbox and being opened since they contain valuable consumer information that is desired and expected.
Yet, realizing this potential takes more than slapping a pretty face (HTML) on your plain text transactional e-mails. It also takes more than balancing your transactional and marketing content, satisfying CAN-SPAM requirements and applying other best practice tips. 
The core challenge comes down to whether you have the right infrastructure to generate transactional e-mail with the right marketing mess