Adding some additional clarification for myself, and anyone else who might read this thread. Users and members are separate, but related. From the docs: https://docs.mailman3.org/projects/mailman/en/latest/src/mailman/model/docs/...
Users represent people in Mailman, members represent subscriptions. Users control email addresses, and rosters are collections of members. A member ties a subscribed email address to a role, such as member, administrator, or moderator. Even non-members are represented by a roster.
Which tells me that the moderation_action property is applied to a member and not a user. So for my use case it appears that I would need to take the following steps. Thus a single **user** record can be associated with multiple **member** records.
- Create a **user** record, if no user already exists
- Create a membership for that email, with the role set to
nonmemberand themoderation_actionproperty set todeferas indicated by Mark earlier in this thread.