Complaint rate is the percentage of recipients who flag your emails as spam. You need to measure your complaint rate on a weekly basis. There are five main reasons for why your complaint rate may be high;
Purchased email lists,
Not including an unsubscribe link,
Incorrect email addresses,
Email sending frequency.
Complaint Rate Calculation:
Total Number of Complaints/Number of Emails Delivered
You can decrease your complaint rate by following the tips given below;
Create a win-back campaign,
Regularly remove inactive users from your list,
Set expectations in your customers' mind,
Make it easy to unsubscribe,
Use double opt-in.