Behavioural retargetting is a technology that's recently been popularised on the web - it's the process of targeting users via banner ad networks after they've visited a brand's site, with an aim to get them to return and convert.This triggered campaign from UK retailer John Lewis uses this methodology in email.

I received the email a few hours after browsing the John Lewis site for iPad accessories, reminding me to go back to the site and providing more reasons to shop with them.As with the retargetting ads online, it's easy to go overboard with this kind of thing, so it's important to pay attention to the tone of voice of the message.

John Lewis do a good job here, gently reminding the user why they're receiving the email but not being too pushy.