Add an Office 365 shared mailbox as IMAP account

Send and receive email using a different email address, with the option of the mailbox being shared with different people.

This solution will work with:

• non-Outlook desktop software (Thunderbird, for example)
• Outlook desktop client, as an additional account
• Microsoft Online - Outlook Web Access (https://portal.office.com)

Requirements:

• Existing Office 365 licensed mailbox user, with one of the following licenses...
  • Office 365 Business Essentials
  • Office 365 Business Premium
  • Office 365 Enterprise (E3 or E5)
• Admin access to the "Exchange admin center" (via https://portal.office.com)
• Full email address of the shared mailbox (if already exists; if not, create new)
• Full email address of the existing Office 365 user
• Password for the existing Office 365 user

Step 1: Create and authorize users to access a Shared Mailbox

From the Exchange admin center click "Shared" under the heading Recipients. Create or edit the shared mailbox, with at minimum the following attributes:

• General: choose the unique email address
• Mailbox Delegation: add at least one existing user with "Full Access" permissions, and at least one existing user with "Send As" permissions
• Email address: add any other aliases, if required
• Mailbox features: verify that IMAP is enabled

Use the shared mailbox with Outlook

Any existing user with "Send As" permission or "Full Access" with see the additional account listed below their existing mailbox when the re-start Outlook. If Microsoft Outlook is already connected to an Exchange account (Office 365 account), then it is not possible to "add" a 2nd account, rather the "shared mailboxes" appear in the same account - listed below the main account mailbox. This is by design, and requires no special settings by the user. Full description of how this works is explained here: https://support.office.com/en-us/article/Open-and-use-a-shared-mailbox-in-Outlook-2016-and-Outlook-2013-d94a8e9e-21f1-4240-808b-de9c9c088afd

Outlook Web Access

Click the icon of the currently logged in user (top-right), and choose Open another mailbox. Type the email address of the shared mailbox and notice that a second browser tab appears with all of the familiar options. The user can now operate both their own mailbox and the shared mailbox.

Non-Outlook email clients

To add an Exchange Shared Mailbox to non-Microsoft email clients or even a "userless" device (printer/CCTV/and so on), use the following IMAP settings:

• EMAIL ADDRESS: shared-name@companyX.com (shared mailbox)
• PASSWORD: (use the password of the user's main account - userX)
• IMAP SERVER: outlook.office365.com (port 993SSL | port 995 = POP3)
  • IMAP USERNAME: 
    • userX@companyX.com\shared-name@companyX.com
      ...or...
    • userX@companyX.com\shared-name
      (Depending upon Excahnge Online version and client device, either of the 2 above usernames may work)
  • (The format is: "user\shared mailbox" - please note the TWO email addresses separated using a backslash)
• SMTP SERVER: smtp.office365.com (port 587TLS)
  • SMTP USERNAME:
    • userX@companyX.com
      
      

The shared mailbox should then appear as a separate account underneath the user's main account. They should be able to receive and send using the shared account.

Further suggestions...

Alert a specific user of new inbound mail to the shared mailbox: You may want to log back into Exchange admin center and go to Mail Flow > Rules, and set up a forwarding rule where email addressed to the shared mailbox is also forwarded to a user's main mailbox; this helps alert the user to the presence of new email without them needing to click out of their Inbox folder.

Save copies of any mail that users send via the shared mailbox: Log onto Outlook Web Access using and open the shared mailbox. Go to App Settings > Mail and under the heading Mail > Automatic Processing > Inbox and sweep rules > add a new rule:

Condition: It includes these words in the senders address... [type the email address of the shared mailbox]

Action: Move the message to folder...[Sent items]