In order to start a new topic from an email, the email must be sent to the forum category it relates to.
ADUG has some new email addresses specifically for the categories that are supported fully in mailing list mode.
Send your post to one of these category specific addresses:
Emails sent from Discourse have a token that identifies the category and topic. When replying to an email from Discourse, Reply List to reply using the token so your post can be placed in the proper location in the forum.