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If the project has no user assigned in Project Editor → General info → Project Manager, then emails are sent to all client's PMs and LPMs.
If in Projects → Project editor → Workflow, we uncheck the option Email users assigned and then update the project, nobody gets an email about this update.
The project's Project Manager who performs an action on project does not receive an email.
The Customer's Project watcher who performs an action on project does receive an email.
The Customer’s Project Manager does not receive emails. They just become the project's Project Manager if the project creator is LPM/CPM.