Emails sent via merge with Word and Outlook (classic) are stuck in outbox

I fixed the issue by disabling the following COM Add-ins:

• Acrobat PDF Maker Office COM Addin
• Adobe Document Cloud for Microsoft Outlook - Acrobat

Based on the symptoms, this is an “email stuck in Outbox” issue in classic Outlook. Use the following checks and fixes that apply to Outlook for Microsoft 365:

1. Confirm Outlook is actually online
   • In the status bar, check that it does not show Disconnected, Working Offline, or Trying to connect….
   • If it does, go to Send/Receive tab > Work Offline to toggle back online, then try Send/Receive again.
2. Check for connectivity and size limits
   • Verify that the internet connection is working by opening a browser and visiting a site such as http://www.microsoft.com or http://www.bing.com.
   • In Outbox, open the oldest stuck message and confirm it does not have a very large attachment (typically >20 MB). If it does, remove or reduce the attachment size, or replace it with a link, then send again.
3. Fix “email stuck because Outlook is offline/Cached mode + registry setting”
   • If messages send correctly when Outlook is started in Safe Mode but get stuck in normal mode, the issue may be caused by the LoadTransportProviders registry value when using Cached Exchange Mode.
   • Workaround (temporary): create and use an Online mode profile for the Exchange mailbox so messages are sent directly without being cached.
   • Permanent fix (requires care with Registry Editor):
     1. Close Outlook.
     2. Open Registry Editor.
     3. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\<x.0>\Outlook\Preferences (replace <x.0> with the Outlook version, for example 16.0 for Microsoft 365).
     4. In the right pane, locate the LoadTransportProviders DWORD value, right‑click it, and select Delete.
     5. Exit Registry Editor and restart Outlook, then test sending again.
4. Remove any hidden/nonexistent stuck items in Outbox
   • If Outlook continues to behave as if something is in the Outbox even when it appears empty or messages keep re‑sticking, use the MFCMAPI tool to remove hidden stuck items:
     1. Close Outlook.
     2. Download and install the MFCMAPI tool (use the 64‑bit build with 64‑bit Outlook).
     3. Open MFCMAPI and choose Session > Logon, then select the Outlook profile.
     4. Double‑click the affected account (cylindrical icon).
     5. Right‑click Root Mailbox > Open contents table.
     6. Look for items with subject such as Read or any message that appears to be a stuck submission.
     7. Select the item, choose Actions > Submit > Abort submit.
     8. Right‑click the item and choose Permanent delete passing DELETE_HARD_DELETE (unrecoverable).
     9. Repeat for any other stuck items, then restart Outlook and test the mail merge again.

If, after these steps, mail‑merge messages still move to a “None” sent date and remain stuck whenever they are opened, collect details (whether Safe Mode works, account type, and any add‑ins in use) and open a support case so Microsoft can review the specific profile and mailbox configuration.

References:

• I can't send or receive messages in Outlook
• Outlook email stuck
• Outlook keeps trying to send a nonexistent message while Outbox folder shows zero messages
• Outlook sync issues cause emails to be stuck in the Outbox and undeliverable mail items