Why you might be getting the error "Mismatching Versions" when installing/upgrading Email-to-anything
This error occurs when you try to install the latest version of an app (e2a) onto an instance that hasn't been updated to the newest Salesforce release yet. This is an expected behavior.
Note: Salesforce Sandboxes instances will not stay with same as Production. If your Sandbox is on a different instance which is upgraded to the latest version you should be able to install e2a on that Sandbox.
You can find information on which instance have been updated on Salesforce Status website: https://status.salesforce.com/status
Please contact firstname.lastname@example.org for any questions.