Once a subscriber has been onboarded, continually messaging them with relevant content is key in keeping them interested in receiving your emails. Start by optimizing subject lines, which are typically the determining factor in whether a recipient opens a message. Subject lines are amongst the most widely tested components of an email message. Test subject lines at the start of a campaign and deploy the winner to the rest of the list. Test such variables as length (be mindful of truncation, particularly for mobile audiences), personalization and positioning of value statements. In your subject lines, consider using words from the most popular searches on your website as well as referring terms from search engines to help keep your messaging relevant. Also, mine the subject lines of your closest competitors for ideas.