If one of JIRA's users has its email address equals to a mail server's username, within the 'Incoming Mail' screen, the mail handler that uses this mail server doesn't work properly, specifically the 'CC Assignee' option which doesn't assign the created issue to the user whose email was on the CC field of the email sent to JIRA.
JIRA also sends an error email to the mail handler's 'Forward Email' with the following error message:

Message appears to have been sent by JIRA. JIRA's mail handler is configured to forward these messages to avoid mail loops.

Steps to reproduce:

1. Configure a new mail server
2. Add a new user or edit an existing one and set this user's email address (within JIRA's User Management) equals to the same username used to set the new mail server
3. Configure a mail handler using the new mail server and check the 'CC Assignee' option
4. Send an email to the mail server's username and add a CC to this email (which is an existing JIRA user)

Note
Even if the user does not have any access to JIRA and is not set to be an assignable user, the incoming mail handler can still set the user as the assignee, see related bug: JRASERVER-65403

• This is due to granting "Assignable User" permission to "Application User - Any Logged in User"