On upgrading the app to an up to date version, it's recommended to upload the app in order to maintain active the configured Approval Mail Handler.

Consider that on uninstalling and installing the app it's possible that configured Approval Mail Handler doesn't register so that approval via e-mail doesn't work.

If you encounter this problem you need to delete the existing Mail System configuration and re-create it as detailed on Configure Jira application emails to allow approval response via email .