Exchange Attachments Stuck in Outbox? A Surprising Fix You Haven't Tried

Has this happened to you? A user calls, frustrated: "I can't send an email with a PDF attachment. It's just stuck in my Outbox."

You immediately jump into troubleshooting mode. "How big is the attachment?" (It's only 2MB). "Is your internet down?" (No, of course not). "Have you tried sending it to someone else?" (Yes, it fails no matter who the recipient is).

Then comes the strangest part: the user can send text-only emails just fine, but the moment they add a simple .pdf or .docx file, the email gets stuck. Or maybe *no* attachments work at all, but text emails fly out without a problem.

The panic starts to set in. You check the Exchange server's event logs, you open the Queue Viewer, and you see it: the "Submission" or "Outgoing" queue is filling up. Hundreds of messages are stuck in a "Retry" state.

Checking the Usual Suspects (And Wasting Time)

At this point, you've probably already gone down the usual rabbit hole:

• You've disabled the antivirus on the server and the client. (Didn't work.)
• You've meticulously checked the firewall logs. (Nothing is being blocked.)
• You've reviewed your Exchange mail flow rules and message size limits. (Everything looks perfect.)
• You've even recreated the user's Outlook profile. (Still broken.)

Stop. The problem is almost certainly not in any of those places. The real culprit is hiding where you least expect it, and it has nothing to do with your mail flow settings.

The Real Reason Your Attachments Are Stuck

The answer is often hiding in the most basic, "no, it couldn't be that" place of all: your server's disk space.

At first, it doesn't make sense. "If the disk was full," you think, "the whole server would crash! Why would it only affect emails with attachments?"

The answer is a smart, self-preservation mechanism in Microsoft Exchange called "Back Pressure."

Here’s how it works:

1. Exchange constantly monitors the free space on the disk where its database or transaction logs are stored (usually the C: drive).
2. When free space drops below a critical level (e.g., less than 10%), Exchange activates Back Pressure to protect itself from a complete crash.
3. One of the first things it does is stop the service that handles the heaviest load: the Transport service. This service needs temporary disk space to process messages, especially attachments.
4. When Exchange can't write these temporary files, it simply stops accepting and processing new messages that require that space. It's the server's way of screaming, "I can't take any more! Clear some space first!"

This is why tiny, text-only emails (which require almost no temp space) might still get through, while even a small PDF (which must be processed) gets stuck in the queue.

The Emergency Fix: How to Get Exchange Breathing Again

If you've RDP'd into your server and see that the C: drive is glowing red, you've found your culprit.

You need to free up space immediately. But wait! Do NOT manually delete any Exchange Transaction Log files (files ending in .log or .jrs)! This is the fastest way to corrupt your database.

Instead, here are the safest places to clear out gigabytes of junk:

• The Biggest Culprit: IIS Logs
  Navigate to C:\inetpub\logs\LogFiles. You will likely find gigabytes of old web logs in the W3SVC folders that can be safely deleted.
• Windows Temp Folders
  Clear out the contents of C:\Windows\Temp.
• Exchange Diagnostic Logs (NOT Transaction Logs!)
  Look in C:\Program Files\Microsoft\Exchange Server\V15\Logging. You can often delete older diagnostic logs (like OWA, ECP logs, etc.) without any risk.

After you've cleared at least a few gigabytes of space, open the Exchange Management Shell and restart the transport service with this command:

Restart-Service MSExchangeTransport

Wait a minute or two. Now, check your Queue Viewer. You should see that massive, clogged-up queue start to melt away as all the stuck emails finally fly out of the server.

The Permanent Solution (So This Never Happens Again)

This emergency fix got you running, but to stop it from happening again, the best professional solution is to move your Exchange "Queue" database from the C: drive to a larger, dedicated D: or E: drive.

Remember, server management isn't just about setup; it's about monitoring and anticipating these stealthy problems before they become disasters.

If you'd rather focus on your business than worry about disk space and transport queues, you should know that Erkmenhost's Managed Server solutions are designed for exactly this. We monitor your server 24/7 to prevent these nightmares from happening in the first place.