We receive this message anytime the Jira mail handler fails to add an attachment to an issue. This is a known problem (annoying), but the question is where is the user configured for this email? In our current instance, there is an "anonymous" user that the prior Jira admin had set up, and the error email notification comes from this user. You can see it in the logs too:
atlassian-jira-incoming-mail.log.2:2016-09-08 12:41:16,686 ERROR [JIRA Admin Mailbox] Caesium-1-4 anonymous Jira Admin email in Exception while adding attachments to REQ-426. Some attachments from the message might be missing. Making a note on the issue.
I can't figure out where anonymous is set. Anyone?