This is such a useful document and I know a great deal of work has gone into it. I hesitate to ask for more, however... I have some suggestions.
1. Section 26.3 Bounce, bouncing
Consider also mentioning "system bounces" if a message cannot be delivered to a group due to a group policy or system limit. For example, sections 4.6.4 (Hashtag permissions), 4.6.13 (Storage Limit Reached), 4.8.6 (Message that contain attachments), etc.
Repeat table headings when a table is split over pages.
3. Formatting topmost outline level
Use outline notation with ending period at the top outline level. For example, use "2. Conventions used in this documentation" rather than "2 Conventions used in this documentation". This helps to visualize and emphasize the outline levels of the document.
Consider using page breaks to separate changes in top outline level.
4. Page headers
Consider adding a header to all pages which contains the current top level (single digit) heading of that page. For example, the header on Section 4 pages 13-29 would be "4. Customizing Group Settings." This helps for wayfinding and context for the reader to know where they are within the document.
5. Apparent links within the document to Glossary items do not work. (formatting tweaks?)
6. Minor cleanup
Replace subscriber with member in sections 4.8.2, 4.8.3, 4.9.1, etc.
Thank you for producing this wonderful Owners Manual!