So we have a number of people who have left over time.. But their Mail and To do still appear in other users drop downs to "open Mail for xyz" or "open to do for xyz".
How do your removed them from the drop down lists.
Hope you have deleted these users from domino directory along with their mail files.
To those user's still seeing other's mail, in their mail file view > More > Preferences > Access&Delegation tab > Shortcuts to Other's Mail. Empty/clear out this field, if there's any name listed.
Then they will be asked to close and re-open their mail file for their changes to take effect. Hope this helps.