Outlook issue

Test the same attachment for the same mailbox in both Outlook on the web and new Outlook, because failure in both clients requires browser/network and Microsoft 365 service or mailbox-policy investigation rather than another application repair.

Since the file opens in another application and restart, repair, reset, location, and corruption checks are complete, do not repeat those steps.

1. Isolate the affected scope

1. Open Outlook on the web in an InPrivate/Incognito window and test the same attachment.
   • If it works, clear the normal browser profile’s cached site data or reset that browser profile.
   • If it still fails, try another supported browser. This test distinguishes browser-session problems from mailbox/service problems.^1^
2. Test from another network, such as a mobile hotspot.
   • If the hotspot works, investigate the corporate proxy, firewall, web filtering, or SSL inspection.
   • If both networks fail, continue below.^1^
3. Establish whether the problem affects:
   • One user or multiple users.
   • One attachment type or all types.
   • Old and new messages.
   • Internal and external senders.
   • Both new Outlook and Outlook on the web.

If one mailbox fails across browsers, devices, networks, and both clients while other users work, treat it as a likely mailbox/service-side issue and escalate with those test results.^2^

2. Use an immediate workaround

From the attachment menu, try Download or Save to OneDrive, then open the saved file. Microsoft documents both actions for Outlook on the web and new Outlook.^3^

If available, use classic Outlook to download the attachment while the web/new Outlook path is investigated.

3. Check Microsoft 365 service health

A Microsoft 365 administrator should open:

Microsoft 365 admin center → Health → Service health → Exchange Online

Review both active incidents and recent issue history. Service Support admin and Helpdesk admin roles can also view service health.^4^

If an attachment-related incident is listed, monitor that incident instead of changing mailbox policies.

4. Conditional administrator checks

If this is a shared or additional mailbox

This branch applies only when the mailbox was opened using Open another mailbox. As a workaround:

• Open the attachment through classic Outlook, or
• Append ?layout=light to that additional mailbox’s OWA URL.

Microsoft documents this behavior for affected Exchange Server 2016/2019 builds; those environments should also be updated to the stated or later cumulative update.^5^

If the error says the attachment is blocked

Check the file extension. Outlook intentionally blocks potentially unsafe types such as .exe, .bat, .vbs, and .js.^6^

An Exchange administrator can identify the OWA mailbox policy assigned to the affected mailbox and adjust that specific policy’s blocked and allowed file/MIME lists. Do not modify every policy unless that tenant-wide scope is intentional. Policy changes can weaken attachment security and may take several minutes to apply.^7^

If policy changes are prohibited, send the file through OneDrive as a link or use the documented compressed-file workaround.

If private-computer WAC viewing is enforced

Only after confirming that the affected mailbox uses an OWA policy with private-computer WAC viewing enforced, an Exchange administrator can change that assigned policy:

Set-OwaMailboxPolicy -Identity "<assigned-policy>" `
  -ForceWacViewingFirstOnPrivateComputers $false

Allow time for policy replication, then sign out of Outlook on the web, sign back in, and retest. Do not apply this broadly without identifying the mailbox’s assigned policy.^7^

If the issue remains after cross-browser, cross-network, and service-health checks, open a Microsoft 365 support request with the affected mailbox, UTC timestamp, attachment type, screenshot, and results from each client/network test.

References

1. Outlook web attachment not open and can't download - Microsoft Q&A
2. Account from my tenant can't see or download attachments on outlook - Microsoft Q&A
3. Open, save, and edit attachments received in Outlook | Microsoft Support
4. How to check Microsoft 365 service health
5. Attachments can’t be downloaded or previewed from Outlook Web App | Microsoft Support
6. Blocked attachments in Outlook | Microsoft Support
7. Can't open or view attachments in Outlook on the web