a simple before/after mockup showing a cluttered inbox (unread, unsorted, mixed priorities) vs. an organized one (labeled, folders, flagged priorities). This visually sells the exact service you offer.
a clean one-page template of the kind of summary you'd send a client each week (e.g., "12 emails handled, 3 flagged for you, 2 meetings scheduled"). This shows clients what ongoing communication with you looks like.