Since the introduction of multiple email addresses in JSM projects:

• https://support.atlassian.com/jira-service-management-cloud/docs/connect-multiple-email-addresses-to-your-service-project/
• https://jira.atlassian.com/browse/JSDCLOUD-798

You need to have one email that will be the default 'Reply-to' address. The option to remove/delete that address will be grayed out until you select a different email as Reply-to:

This causes some confusion. It would be good to have some kind of warning on the screen indicating that you can't remove an address that is set as the default Reply-to.