> ## Documentation Index
> Fetch the complete documentation index at: https://docs.tenantcore.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Open a Mailbox in Outlook

> Use a licensed Mailbox Access Account to open TenantCore-created mailboxes in Outlook.

# Open a Mailbox in Outlook

TenantCore can give a licensed Exchange Online user delegated access to TenantCore-created mailboxes.

This lets you work with replies in Outlook without assigning a separate Microsoft licence to every sending mailbox.

## Before you begin

Your BYOT tenant needs an eligible licensed Exchange Online user.

That user becomes the **Mailbox Access Account**.

## Open the mailbox workflow

1. Open **Mailboxes**.
2. Choose **Open Mailbox** for the mailbox you want to access.
3. Select an eligible licensed user.
4. TenantCore begins assigning access.

TenantCore grants:

* **Full Access**
* **Send As**

## Wait for permission verification

The UI can show **Assigning access...** while Microsoft applies the permissions.

Do not assume the permissions are ready just because Microsoft accepted the initial request.

TenantCore verifies the delegated state before treating the mailbox as ready.

## Sign in correctly

When Outlook opens, sign in with the **Mailbox Access Account** you selected.

The Mailbox Access Account is the licensed Microsoft user.

The TenantCore-created mailbox remains the mailbox identity you are opening and sending as.

## If Outlook does not open the mailbox

Microsoft delegation can take time to propagate.

If access does not work:

* confirm you signed in with the assigned access account
* wait for Microsoft propagation
* return to TenantCore and check delegation status
* use the repair/reapply action if available
* confirm the selected user still has a valid Exchange licence

## Changing the access account

For BYOT tenants, TenantCore can change the selected Mailbox Access Account.

The safer sequence is:

1. grant and verify access for the new user
2. confirm it is healthy
3. remove the old delegation

This avoids creating an unnecessary access gap.