Office 365 Shared Mailbox Emails Are Stuck in the Outbox? Fix it

Are you facing issues where Office 365 shared mailbox emails are stuck in the Outbox and won’t deliver? I have seen this occur many times in user accounts. It is mainly because they lack the “Send As” permission.

In this tutorial, I discuss the common troubleshooting tactics you can use to fix the issue yourself.

What is the Outbox? Where Shared Mailbox Messages Get Stuck?

Outbox is a temporary folder hidden from view. When you are sending a message, the Office 365 email puts the message there. If all the criteria and permissions match your digital mail carrier, take the message to the destination.

However, the message gets stuck in your own personal Outbox. It never leaves your application because the server stops it.

Why Are My Office 365 Shared Mailbox Emails Stuck in the Outbox?

The message you are sending is larger than the maximum permissible size. By default, the message size constraint for a Shared mailbox is:

• 35840 KB for Sent messages.
• 36864 KB for Received messages.

Although administrators can edit these, they can only choose a value that lies between 0 and 153600 KB  ~ 150 MB.

If your message exceeds the value the admin has set, then Office 365 Shared Mailbox emails are stuck in the Outbox.

Office 365 Shared Mailbox emails are also found to be stuck in the Outbox when the local OST file gets damaged or corrupted. Outlook fails to process the outgoing mail.

Another common scenario is that there are too many people trying to access/use the shared mailbox simultaneously. When more than 25 people attempt to send emails from the same shared mailbox address. It puts some of the messages in the Outbox queue to be sent when there is less strain on the mailbox.

During the Shared mailbox assignment, the administrator deliberately gave you just the “Read and manage” permissions. In other words, you lack the “Full access” permission required to send messages from any shared mailbox.

To add the correct Permissions, we have 3 methods; use the one that you are most comfortable with.

Method 1: Using the Microsoft 365 Admin Center

On the main Microsoft 365 Admin Center page, the steps are as follows:

• Step 1. Log in to the admin center, expand the Teams & groups tab from the left-hand pane, and select Shared mailboxes.
• Step 2. Search for and select the particular mailbox for which you are assigning permissions.
• Step 3. In the pane that appears, you will see categories for mailbox permissions. Add the user’s email address to the required categories:
  • Read and manage permissions
  • Send as permissions
  • Send on behalf of permissions (Optional but recommended)
• Step 4. Click on the permission type you wish to grant, then click + Add permissions.
• Step 5. Select the user’s name and click Add.

The fixes to Office 365 Shared Mailbox emails are stuck in the Outbox, won’t be immediate, wait for a while, and also ask the user to log in again/refresh their browser/or close and reopen the Outlook client.

Method 2: Using the Exchange Admin Center (EAC)

Here is what your admin needs to do if they choose the EAC route:

• Step 1. Log in to the Exchange Admin Center, expand Recipients, and select Mailboxes.
• Step 2. Use the filter to display only Shared Mailboxes.
• Step 3. Select the shared mailbox you want to manage.
• Step 4. Switch to the Delegation tab and click Edit under Read and manage (Full Access).
• Step 5. Click + Add members.
• Step 6. Search or scroll for the user account(s), mark the checkbox next to their name, and click Save.
• Step 7. Wait for a few minutes for the role assignment to finish. Then, ask the user to sign out and sign back in before sending an email again

Method 3: PowerShell to Fix Office 365 Shared Mailbox Emails are stuck in the Outbox

I have seen some users complain that reassigning permissions using the admin center doesn’t make any difference. So we suggest a more powerful way to make the permission reassignment. These are PowerShell commands.

Step 1. Connect a local PowerShell instance with the Exchange Online environment.

Step 2. Then, remove any existing permissions (we do this as they are not working for some reason):

Remove-MailboxPermission -Identity “[email protected]” -User “[email protected]” -AccessRights FullAccess

Use this command in particular to give the permissions that can release Office 365 Shared Mailbox emails stuck in the Outbox:

Add-MailboxPermission -Identity “[email protected]” -User “[email protected]” -AccessRights FullAccess -AutoMapping:$false

Make sure you replace the placeholder variables with your own credentials.

We keep the “-AutoMapping” flag off to prevent Outlook from starting its own request, as this is a major source of the issue.

Instead, what we do is add the same Shared Mailbox to users’ classic client manually:

File > Account Settings > Change > More Settings > Advanced > Open these additional mailboxes.

Try and send an email as a test. Make sure you pull the “From address” from the Online Global Address List to prevent any cache issues. Moreover, it is possible that your Office 365 Shared Mailbox emails are stuck in the Outbox, not because you lack the permissions, but because there is something else that isn’t working. Most of the time, it is the local client.

Method 4: Fix Local Profile Corruption in O365 Clients.

Both new and classic editions of the Office 365 desktop email client use local profiles to manage your email on your device.

If any external factors, like a faulty add-on or a virus/malware attack on your device, can damage it, try these steps to remove a stuck email from the Outbox of Outlook

Best Practices When Office 365 Shared Mailbox Emails Are Stuck in the Outbox

Admins should limit the shared mailbox access to 2-5 members, not more.

Users should be aware of the per-email size limits and draft their emails accordingly. Plus, they should pause all mail activity on that particular shared mailbox.

Also, Shared Mailboxes have a non-expanding 50 GB cap. When this maximum capacity is reached, it becomes impossible to send new emails. You have to delete some old conversations to make space for new ones.

Instead of deleting, you can use the SysTools Office 365 Backup and Restore Utility. This tool allows you to save a copy of all your official conversations that took place over the Office 365 shared mailbox. Download and use the tool.

Sometimes, if the issues fail to be fixed even after multiple attempts, admins may have to take a call to delete and create a new Shared Mailbox in Office 365 again.

Conclusion

Here we saw what to do if you see Office 365 Shared Mailbox emails are stuck in the Outbox. Just fix the exact issue that is causing the problem: Get the Full access + send on behalf permissions. Keep message size manageable, communicate with other users so that shared mailbox access does not clash, and of course, keep your OST file healthy. Once you carry out these tasks, emails should not get stuck.