The following should be conducted in a test environment.
Configure a ScriptRunner Mail Handler for Local Files (see 2021-01-13_14-12-00_Add Incoming Mail Handler).
Copy or write an email message in the <jira-home>/import/mail folder.
Start with a simple script such as:
Click the Run Now.
A Script executed successfully notification appears if the mail handler ran as expected. If there is an error, a Cannot handle message notification appears. In the case of an unsuccessful execution, select More Details link to get more information.
Once tested, add the configuration to your Jira instance.