Mail and Calendar app received a new update for Windows 10 Mobile devices. The latest update brings back the linked inboxes feature which was pulled temporarily to fix a bug.
Along with linked inboxes, the latest update also includes new settings pages, options are separate like “Swipe Actions”, “Notifications”, Signature etc.
For linking the inboxes you need to go to manage accounts and tap on link inboxes. Then you can choose which accounts to link and can give a name to new inbox. We named it universal as can be seen in the their screenshot above. You can go back to Manage accounts and tap on the new unified account name if you wish to unlink any of the accounts.