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When adding a customer to a project and this error appears, usually it means that the customer is inactive on the site.
Can you please ask your site administrator to go User management > Jira Service Desk and confirm if the affected customer is active?
When you try to add the email ids which are in a deactivated state in JIRA Software, there is an issue.
The solution is to activate the user account in JIRA Software and then add the email to the JSD organization. It will work. Then you can deactivate back the email id in the Jira Software account.
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