If you interact with your audience through the online medium then chances are that you already use triggered messages in the form of transactional notifications such as a welcome message, a thank you email, or a purchase confirmation email. It is well understood by most online marketers that these welcome and purchase confirmation messages have very high open rates (sometimes over 70%) and while there is no stated or unstated stipulation that these messages have to be free of marketing messages, they are generally just plain transactional messages.
So always consider placing a marketing message and a call to action in your transactional notification. For example, a purchase confirmation could include offers for complementary products or services, a thank you message could also promote your "refer-a-friend and earn additional rewards" program. A little planning will help you determine which marketing message and which call to action to include.
Source: Bluestreak's "BlueNotes" newsletter.