I get an error: Could not create request on behalf of the sender
Creating service request fails and you can find "Could not create request on behalf on behalf of the sender." in the Incoming Log. The request could not be created in the name of the reporter.
The email you are sending mail from once had a customer account on JSD and was deleted, but still has requests reported by them. Jira didn't remove the user fully, that's why it cannot create a new one, but also cannot reactivate the old one.
Check if there is a request with the sender's email as the reporter and there is no customer with this email on Administration>User management>Jira Service Management>Portal only customers.
Follow these steps to fully remove the user from Jira:
Remove the sender from the requests where they are the reporter.