Actually, it seems that we should look at this instead:
Discourse creates staged user accounts for posts coming from mailing lists with email addresses not found in the system (example). This means that we cannot add a second email address to @TheDJ because it is already taken by the staged user “Derk-Jan_Hartman”. Instead, “Derk-Jan_Hartman” should be merged into @TheDJ.
What it is not clear to me is what is the end result. Do we have one user with two email addresses assigned, allowing new MediaWiki-l emails to be correctly assigned to the new user? Or is this a one-time merger, the second email address is lost, and a new staged user will be created when a new email from MediaWiki-l arrives?
It worked! Now your post to MediaWiki-l appears authored by @TheDJ and it is reflected in your user profile.
In the admin view of your user profile you have now a primary address (the one you registered with here) and a secondary one (the one you use for MediaWiki-l. Therefore, future emails sent to MediaWiki-l should just appear here as sent by the one and only @TheDJ.