After git tag foo && git push --tags the commit message produced by git-commit-notifier got stuck in mailman's moderation queue.
Closer inspection revealed that it was sent with a From: [email protected] header instead of the expected From: Marius Gedminas <[email protected]>.
If git-commit-notifier doesn't have enough metadata (this is a lightweight tag after all) and has to fall back to the Unix user account who ran the post-receive hook, it could at least use that user's ~/.gitconfig to get appropriate user.name and user.email.