First, send to the right people. Don’t buy/rent/steal email lists. Those people are not your customers and they never agreed to receive anything from you.
Make your own email list. It’s the harder way, but giving you their email address, your customers agree to receivе emails from you.
Beware, people often sign up for invalid emails so include email verification in your process –or validate with a bit of code (one @ symbol, proper domain, etc.). It’s a good idea to revise your list, too, if you have a sound record of inactive recipients.
These steps will give you a nice, active and real recipients list. This means fewer spam complaints and less unsubscribes.
Then, make sure you answer yes to the following questions:
Is your campaign well-targeted? Is your subject line appealing enough? Is your sender address/ name trustworthy?
Was your campaign well-timed and relevant to customers’ history with you? Is your offer appropriate for their preferences – average order value, location, demographics, etc.?
Craft campaigns carefully and try to provide real value instead of just bombard with promo codes. That can actually hurt your brand and cause people to buy at discount only.