Office 365 Shared Mailbox Emails Stuck in Outbox? Here’s What Works for Me

I’ve seen Office 365 shared mailbox emails stuck in Outbox many times, and it’s usually caused by permission issues or Outlook glitches. Here’s how I troubleshoot and fix it.

What is a Shared Mailbox in Office 365?

Before I dive into the emails stuck in outbox shared mailbox issue, let me quickly explain what a shared mailbox is, just in case you’re new to managing Office 365.

A shared mailbox allows multiple users to read and send emails from a common email address, like support@exampleorganization.com or sales@exampleorganization.com. It is mainly used for handling customer queries, internal communication, or project collaboration.

Shared mailboxes don’t have login credentials. Users with appropriate Send As or Send on Behalf permissions can access them.

Why are Office 365 Shared Mailbox Emails Stuck in Outbox?

When I encounter emails stuck in the Outbox of a shared mailbox, it’s usually because of one or more of these common causes:

#1. Missing or Incorrect Permissions

If you don’t have the correct permissions on the shared mailbox, Outlook or Outlook on the web won’t allow you to send emails, leaving them stuck in the Outbox.

• Send As Permission
• Send on Behalf Permission
• Full Access Permission

To address Office 365 shared mailbox emails stuck in the Outbox because of this reason, I usually confirm or assign permissions using Microsoft 365 Admin Center or PowerShell. Here’s how I do it:

1. Microsoft 365 Admin Center

• Open https://admin.microsoft.com, and sign in with your admin account.

• Click Teams & groups > Shared mailboxes.

• Find and click the shared mailbox to edit.

• Under Members, click Edit(Full Access).
• Add the users who should have access to open the mailbox.
• Click Save when done.
• Now, assign the Send As Permission, scroll down to Mail flow settings > Send As permissions.

• Click Edit > Add permissions.
• Choose the user(s) to send emails from the shared mailbox.
• Click Save.

• Afterwards, assign the Send on Behalf Permission (Optional)
  • Under Mail flow settings, find Send on behalf permissions.
  • Click Edit > Add permissions.
  • Select the user(s) > Save.

When everything is done, ask the users to restart Outlook or sign out and sign back in to refresh permissions.

2. PowerShell

Add-MailboxPermission -Identity “SharedMailbox@exampledomain.com” -User “User@exampledomain.com” -AccessRights FullAccess

Add-RecipientPermission -Identity “SharedMailbox@exampledomain.com” -Trustee “User@exampledomain.com” -AccessRights SendAs

#2. Outlook Profile Issues

Sometimes, I’ve seen that corrupted Outlook profiles or outdated cached data cause the Office 365 Shared Mailbox Emails Stuck in Outbox problem. To fix, I will:

• Remove and re-add the shared mailbox.
• Create a new Outlook profile to clear the old cache.
• Turn off cached mode for shared mailboxes.

#3. Large Email Attachments

If I try to send an email with large attachments, especially over 25 MB, Outlook or Exchange Online poses issues sending it. The steps I followed to resolve:

• Compress or use OneDrive links instead of attaching large files.
• Check the Send/Receive settings in Outlook for errors.

#4. Autodiscover or DNS Misconfiguration

A misconfigured Autodiscover service can cause authentication and sending problems. The Office 365 shared mailbox emails stuck in the Outbox usually appear when I’m configuring shared mailboxes in hybrid environments or newly added domains. Luckily, I found a way to fix :

• • Verify Autodiscover using Microsoft Remote Connectivity Analyzer.

• Open https://testconnectivity.microsoft.com/

• • Select > Outlook Autodiscover under Exchange tests.
  • Enter Details:

• Email
• Username
• Password

• Run the Test > Click Perform Test.
• Check Results: Success or Failure

• Ensure the right CNAME record for Autodiscover exists in DNS.

#5. Mailbox Quota Reached

Shared mailboxes in Office 365 typically have a 50 GB storage quota by default. If that’s full, no new emails, including outgoing, will be processed, and it shows Office 365 shared mailbox emails stuck in the Outbox error. Here are the steps to resolve: 

• Check mailbox size in Exchange Admin Center (EAC).
• Open https://admin.exchange.microsoft.com, and sign in with the admin account.

• Click Recipients > Mailboxes.

• Scroll through the list or search for the shared mailbox.

• Click on the mailbox name to open its details.

• Under Mailbox usage, you’ll see:
  • Current mailbox size
  • Storage limit/quota
  • Warnings if the mailbox is nearing or has exceeded its limit

• Archive or delete unnecessary emails, or request an increased quota if possible.

Before deleting anything important, I recommend backing up the mailbox data first. I prefer using the SysTools Office 365 Backup tool for this. It helps me export mailbox emails to local storage simultaneously. After the backup, I can delete unwanted emails to clear space.

Related article – Add Rules to a Shared Mailbox in Office 365: Detailed Steps

Author’s Verdict

The Office 365 shared mailbox emails stuck in Outbox issue usually occurs due to a permission misconfiguration, profile corruption, or attachment issues. By following the steps I’ve outlined, you can quickly resolve emails stuck in the outbox shared mailbox error without any technical expertise.

Also Read: How to Migrate Shared Mailbox to Office 365?