You can customize Mail Me app settings for each of your Jira projects. To do this, go to Projects tab on the left.
Here you can:
This will allow users assigned to a certain project use Mail Me app in the selected project.
This will allow Mail Me users in the selected projects to add descriptions when sending a certain Jira issue by email. If this feature is disabled, the users will not be able to add any comments in the description, they will be able to send Jira issue details as is.