The list of novel mediums on which to place an online ad continues to grow, from airline boarding passes to checking account statements. Transactional emails - those emails that confirm a purchase or order or delivery - are another, writes Loren McDonald at Silverpop.

"Transactional emails can also upsell, inform, brand and market to customers just like regular promotional emails, but to more engaged recipients," he said. Problem is, most companies need to improve their service around these emails first. For starters companies could make them prettier, McDonald said, as they tend to be text-based and ugly. They also can be difficult to find in an inbox.

"Many companies rely on homegrown transactional systems or ecommerce platforms that lack the authentication, delivery and bounce processing infrastructure common to email marketing or dedicated transactional email systems," he writes.

Some service providers outside of the email marketing space are beginning to think along these lines as well. Assistly, a customer service company, includes routine transactional emails such as those sent to confirm a sale in its channel. "Our system is configured to treat email from a customer, no matter where it comes from, as one of the service inputs," Matthew Trifiro, senior vice president of marketing, told CRM Buyer.