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.
Test. Different email clients and mobile devices display emails differently. Send test emails to colleagues, or use a testing program to make sure your emails are going to look good on screens big and small. Testing reveals design mistakes before it's too late, and testing programs can predict whether or not a campaign will get caught in a spam filter. You could even set up accounts with a few different email services for easy testing. Avoid sending one big image as a campaign, and cover your bases with a plain-text option for every email.
If you don’t know the conversion rates on your optin page, then setup split testing. I recommend Leadpages for setting up your optin pages because they have built-in click tracking, A-B split testing, and they let you drive unlimited traffic. Of course, they have some beautiful optin page templates that you can edit very easily using their click-click interface.
Alternatively, at least, an eye-catching design. It’s worth letting a proper designer have a go at prettifying your emails. Customers are often opening emails on the go, so you need to work hard to grab their attention. Making your emails nice to look at is a brilliant way to hold their eye. Netflix put time and effort into making their emails stand out, personally and visually – and it pays off for them!