The Creating Issues and Comments from Email document mentions

reporterusername
This sets which user will be the "reporter" of created issues, for emails whose sender does not match that of an existing user. Normally JIRA will ignore emails from addresses not matching an existing user. For instance, to allow anonymous users to create issues via email, you can create an anonymous user or dummy account on JIRA and set the reporterusername to point to this account. When the "reporterusername" parameter is specified, the "from" address of the email is added at the end of the comment of the issue, so you can identify the sender.

It appears that he following line is appended to the description of new issue. Comments, however did not have this appended, therefore it would seem that our documentation (or implementation) is incorrect.

[Created via e-mail received from: xyz@example.com]