Feature estimation supports forecasting value delivery, applying WSJF prioritization, and sizing epics by splitting them into features and summing their individual estimates. Feature estimation usually occurs in the analysis state of the Program Kanban and relies on normalized estimation techniques, similar to the methods used by Agile teams (see the Iteration Planning article for more detail). During analysis, select subject matter experts from the ART engage in exploration activities and preliminary sizing. During the analysis state, sizing features do not require splitting them into stories or the inclusion of all the teams that might develop them.
I usually recommend no more than about 2500 - 5000 messages in any of the critical path folders. The critical path folders are the Calendar, Contacts, Inbox, and Sent Item folder. Ideally, keep the Inbox, Contacts and Calendar to 1000 or less. Other folders, particularly custom folders created by the user, can handle having larger numbers of items without having a broad impact on the user experience (20,000 items in my "Cookie Recipes" folder? No problem - except when I need to find that recipe from last Christmas!).
If getting word out to the users to reduce folder item counts is impractical, administrators have another option. Administrators can use the Mailbox Manager tool to control the size of critical mailbox folders. Unfortunately, Mailbox Manager does not evaluate the mailboxes based on message count within a folder— instead it processes messages by age and/or size of message. Regardless, if your organization allows the use of it, it can help prevent mail folders - and user-frustration - from getting out of control.