It is possible to add a comment to an issue by email without having to log in to the system, but the sender apparently needs to have a JIRA account with an email account that matches the email account of the incoming email. Sending an email to confluence at concord.org with a subject line that contains the issue's key will automatically enter the comment in the issue's comments.
 
The issue key can be seen at the top left corner of an issue's web page under Issue Details. It is created by Jira to serve the same functionality as a database record's unique primary key and is designated by the Project's unique key (the short acronym specified when the project was created) and the chronological issue number for the project. Case is not important when specifying the issue key.
 
This functionality works by using the Jira's service setup for comment emails. This service reads confluence's email account and looks for issue keys in the subject line of all emails, so the email account needs to seldom used and definitely not used within JIRA.