Email notification to a non-Jira user appears as plain text instead of HTML

Our default email format is set to HTML and our notification scheme also notifies a Single Email Address (to a distribution email alias, my.group.of.users@example.com).

For example, a change notification will go to actual Jira users as well as the Single Email Address. The notification to a Jira user arrives as HTML format, but the same notification that goes to the Single Email Address arrives as plain text.

We want the email that goes to the Single Email Address to be sent as HTML as well.

Why does the notification that go to an email address get sent as plain text instead of the nicely formatted HTML version?