To reduce hard bounces, you need to validate your contacts by purchasing the validation credits from the billing section. 
 
Here are the steps to validate your contacts:
 
  • Please go to Contacts
  • Select all the contacts that you want to validate
  • Click on Bulk action and select Validate Emails
 
The validation process will be initiated and it will take around 30 minutes to 1 hour depending on the number of contacts selected. You will get an email once the validation process is completed.
 
Please check the article https://our.eb-help.com/help/articles/emailvalidation to know more about email validation.