It would be useful to be able to assign mailboxes to one or more projects (especially if you manage mailboxes for multiple clients or different use cases—for example, 30 mailboxes for client/project A, 17 mailboxes for client/project B, etc.).

That way, when selecting mailboxes for a campaign, instead of having all mailboxes mixed together in a single list, you could expand a project category via a collapsible menu and select mailboxes within that project.

This would reduce the risk of accidentally including in a campaign mailboxes that are intended to promote a different service.