When downgrading your pricing plan to the START plan, a situation has occurred where you actively use the INBOX feature, which is not available in this plan. It is necessary to deactivate the feature.
In order to deactivate the INBOX, each user must disable it in their User Settings. In the top right corner, click on your name » My Profile » E-mail Settings. First, you need to Disable Synchronization and then click on the red Delete Account button. After that, just confirm the pop-up warning about the consequences of disabling the INBOX and the feature will be deactivated. Once INBOX has been deactivated for all user accounts, you can proceed with switching to the START pricing plan.