Outlook crashes when sending large attachments (over 20MB)

You’re composing an email, attach a file — maybe a PowerPoint deck or a PDF around 25MB — and the moment you click Send, Outlook freezes. The mouse cursor turns into a spinning circle. A few seconds later, Outlook crashes entirely. Or maybe it gives you a vague error: "Outlook has stopped working".

I’ve seen this happen most often with files between 20MB and 35MB on Outlook 2016, 2019, and Microsoft 365, especially on Windows 10 and Windows 11. The crash usually hits right when Outlook tries to compress or encode the attachment before sending.

Why this happens

The root cause is almost always one of two things — or both together:

1. Outlook’s built-in attachment size limit. Microsoft 365 and Outlook 2019 default to a 20MB limit for outgoing attachments. Files above that force Outlook to attempt compression, and that’s where it chokes.
2. A corrupt temporary cache file. Outlook creates a temporary copy of every attachment you send, stored in %temp%. If that temp folder is corrupted, full, or has permissions issues, Outlook crashes when it tries to write to it.

Some people also have an add-in that hooks into the send process (like a PDF converter or CRM plugin). That can trigger the crash too, but the temp folder issue is the one I see nine times out of ten.

The fix: clear the temp folder and compress the file

Skip reinstalling Outlook. Don’t bother with the Office repair tool yet. Try this first.

Step 1 — Close Outlook and clear the corrupt temp folder

1. Close Outlook completely.
2. Press Windows + R, type %temp%, and hit Enter.
3. In the Temp folder that opens, press Ctrl + A to select all files and folders.
4. Press Delete. If some files say they’re in use, skip them — just check the box “Do this for all current items” and click Skip.
5. Empty the Recycle Bin after.

That clears the cached junk.

Step 2 — Restart Outlook and try again

Open Outlook. Compose a new email. Attach the same file. Hit Send. If it works, you’re done. If it still crashes, move to Step 3.

Step 3 — Compress the file before attaching

Right-click the file in File Explorer, choose Send to > Compressed (zipped) folder. Then attach the ZIP file instead. Outlook handles ZIP files much better — no crash, and the recipient can unzip it. This also gets around the 20MB limit because the compressed file is often under 20MB.

If the ZIP is still over 20MB, use a cloud link instead (OneDrive or Google Drive).

Step 4 — Disable add-ins if it still crashes

If compressing the file didn’t help, an add-in is almost certainly the problem.

1. In Outlook, go to File > Options > Add-ins.
2. At the bottom, next to Manage, select COM Add-ins and click Go.
3. Uncheck all add-ins except the ones you know are essential (like Microsoft Exchange Add-in).
4. Click OK and restart Outlook.

Try sending the large attachment again. If it works, re-enable add-ins one at a time until you find the culprit. Common offenders: Adobe Acrobat PDF Maker, Dropbox for Outlook, and any CRM connector.

What if it still fails?

If you’ve cleared the temp folder, compressed the file, and disabled add-ins, but Outlook still crashes, try these three things in order:

• Run Outlook in safe mode: Press Windows + R, type outlook /safe, and hit Enter. If it works in safe mode, a third-party add-in is definitely the problem. Go back to Step 4 and disable everything.
• Check your Exchange or IMAP account settings: Some corporate Exchange servers enforce a hard 25MB limit. If your attachment is over that, Outlook will fail even with a clean temp folder. Use OneDrive or SharePoint to share the file instead.
• Repair Office: Go to Control Panel > Programs and Features, right-click Microsoft Office, choose Change, and select Quick Repair. If that doesn’t help, try Online Repair (takes longer but fixes deeper issues).

I’ve seen this exact problem on dozens of machines, and 90% of the time it’s the temp folder or an add-in. Don’t overthink it. Start with the folder cleanout — it takes 30 seconds and often solves it.