Boomerang adds scheduled sending and the easiest, most integrated email reminders to Gmail, helping you reach Inbox Zero.
How to Cancel Boomerang for Gmail Manually
Unsubscribe by following these steps:
For individual subscriptions, click the Boomerang icon in the upper right corner of your Gmail window. Next, select Manage scheduled messages. Then click Account and under the “Billing Information” section click on the “Don Not Renew” link.