Unable to send mail: JIRA user name / full name not recognized as JIRA user

We cannot send mails to our internal JIRS users and receive following error message: The following recipients could not be identified as JIRA users or email addresses: Stephan,Gramlich
Is there a problem with firstname + space + lastname? Sending mails to our customers only is working, but as soon as one of our JIRA users is selected, the error message appears. We manage our customer accounts in JIRA, our internal JIRA accounts are managed within the active directory.

Thank you for your help