Contacts that haven't opened a single email in the last 6 months are considered inactive.

Email marketing automation software such as EcoSend can identify inactive users. All you need to do is build a 'Smart Group' with filters to define your inactive contacts.

Any inactive contacts can be sent an automatic re-engagement email. If this doesn't generate a response, the contact should be removed from the list.

That's because having a clean and healthy list will deliver a better ROI on any email marketing efforts.