A linked inbox for multiple email accounts in the Mail app was arguably the most requested feature following the release of Windows 10. Indeed, this was also one of our biggest qualms following Windows 10's release. If you are as eager as about a unified inbox as we are, you'll pleased to know that Microsoft has made some solid progress on that front.
The latest update to the Mail client on Windows 10 Mobile brings with it the ability to link multiple inboxes into one unified inbox. To do so, you'll need to access the pivot menu (by clicking the three dots at the bottom), then go to 'Manage Accounts' and select 'Link Inboxes'. This will open up a new dialog that will allow you to choose which of the accounts you want to link and the name for the new unified inbox. Voila! You no longer need to rummage through menus and cycle through different folders to have access to all your email.
Unfortunately, though, the PC version of the app has had no such luck and is still missing this hugely coveted feature. Since Microsoft has moved to the Universal Windows Platform for all its first-party apps, though, the feature will be probably be making its way to the PC soon.