One of the best ways to handle this would be, to ask your subscribers to add you to their address books. Whenever someone subscribes to your newsletters/service, as a first step you either take them to a 'thank you' page or send them a 'welcome' email. It is a good practice to explicitly leave a 'please add us to your address book' message there.
If your subscribers add you to their address books, it means they trust your brand and look forward to receiving emails from you. Not just that, doing this will help improve your deliverability rates as well. You read it right—"deliverability rates"!
Read this week's tip to know an easier and more automated way to ask your subscribers to mark you as a safe sender.