Symptom

Despite correct configurations of a project and mail handlers, the assignee of new issues do not get set as expected when created via a mail handler (whereas it gets set as expected through the web interface).

Cause

This occurs when a JIRA user account is registered as having the same e-mail address that the mail handler itself is using to pull messages from. Essentially, a registered e-mail address of a JIRA user account takes precedence over the 'Default Assignee' value for a project.

Resolution

Ensure each mail handler has a unique e-mail address for which it can receive messages, distinct from the address used by the mail handler.