Some users have multiple e-mail addresses / aliases, e.g. j.brown@example.com, james.brown@example.com etc.
Still it is the same person, who has one JIRA account.
But the comment is created automatically only if the e-mail comes from the address listed in JIRA.

I think it would be a good idea to define an entire list of user's e-mail addresses. One of the addresses would be the main one.
Other would exist to better idetify the user for the automatic issue / comment creation purpose.