The relevance of an email is an extremely important factor that contributes to the succeeding or not succeeding of an email marketing campaign.
Email subscribers have specific expectations when they confirm their registration. Make sure to communicate the subject, topics and frequency on the registration form, so every receiver knows what to expect.
Offering different frequency options can help too since frequency is one of the main reasons why people unsubscribe. When someone subscribes to your email list you should immediately gather his preferences from the very start of the subscription process.