It is possible for an e-mail to create an infinite loop if JIRA picks up its own sent email.

There are a few ways this can occur, but one example is if JIRA checks a box called helpdesk@atlassian.com and an e-mail is sent with the following headers:

To: <helpdesk@atlassian.com>
From: "Viagra.com" <helpdesk@atlassian.com>

then JIRA will send an e-mail back to helpdesk@atlassian.com once the issue is created. It will pickup this e-mail again, and start an infinite loop of creating comments and sending e-mails for this particular issue.

The email handlers in JIRA should be smart enough to check the from address to make sure it isn't the same as the to address.