Offhand, the reason doesn’t seem to stick: the account is active, not suspended nor silenced. However I noticed the account is not “staged”, which is the case for most of the accounts whose mailing list posts do end up in Discourse. All of the posts from non-staged users are from members of the moderation team.
Our mailing list mirror category was initially set up using this guide: Mirroring a read-only mailing list in Discourse
However, I just reviewed the configured permissions on the category and it seems like our moderators group was added, and `everyone` was given `reply` permissions.
I’ve removed these extra permissions and left only `everyone` with the `see` permission. Perhaps these “extra” permissions are interfering with the way Discourse handles these special “mailing list mirror” categories?