Two particularly important groups for our purposes are customers with a one-time purchase and customers who have purchased multiple times. A customer is sometimes not considered to be loyal or repeat customer until they have purchased two to five times, in which case the single purchase segment is more akin to a warm prospect than a loyal customer.
To make sure you're only sending emails to the people who want to read them, clean up your email list so that it excludes recipients who haven't opened a certain amount of emails in the campaign's recent history. This makes sure your emails' open and clickthrough rates reflect only your most interested readers, allowing you to collect more effective data on what is and isn't working in each email you send.
When it comes to deciding how to craft that perfect subject line, there appears to be really only one area to avoid: the subject line of 60 to 70 characters. Marketers refer to this as the “dead zone” of subject length. According to research by Adestra, which tracked over 900 million emails for its report, there is no increase in either open rate or clickthroughs at this 60-to-70 character length of subject line.