Fixes / Outlook · Checked

Outlook blocked access to potentially unsafe attachments: fix it

Short answer

Outlook blocks attachments whose file type can run code, such as .exe, .bat, .js, .vbs, .ps1 or .iso. The file stays in the email but you cannot open, save or print it. The quickest fix is to ask the sender to resend it as a .zip file or as a OneDrive or SharePoint link, since those are not blocked.

What this message means

Outlook sorts file types into security levels. Level 1 file types (programs, scripts, disk images, shortcuts and more than 200 other extensions) are treated as unsafe and blocked completely. When an email arrives with one of these attached, Outlook shows this message at the top of the email and hides the attachment. You cannot open, save, delete or print it. It does not mean the file is infected. Outlook blocks it only because of its file extension. The same rule applies in Microsoft 365, Outlook 2016 to 2024, new Outlook and Outlook.com. Many companies also block extra file types through their Exchange or Microsoft 365 settings.

Common causes

CauseHow to tell
The attachment has a blocked (Level 1) file extension such as .exe, .bat, .cmd, .js, .vbs, .ps1, .msi, .jar, .iso or .vhd.The message names the file, and its extension is a program, script or disk image. The attachment is missing from the attachment bar.
A harmless file got a risky extension, for example a database (.mdb, .accdb), an Outlook data file (.pst), a Python script (.py) or an internet shortcut (.url).You expected a normal work file, but the name in the warning ends in one of these extensions.
Your organization blocks extra file types through an Exchange or Microsoft 365 policy.The same file opens fine on a personal Outlook account, but not in your work mailbox. You may also see it blocked in Outlook on the web.
The attachment is a linked file (attached by reference) created by a custom program or add-in, which Outlook 2010 and later block by default.The email was generated by an internal system or add-in rather than sent by a person, and the blocked item is a link to a file path instead of a real file.

How to fix it

1. Ask the sender to resend the file as a .zip

  1. Reply to the sender and tell them Outlook blocked the attachment.
  2. Ask them to right-click the file in File Explorer and choose Send to > Compressed (zipped) folder (on a Mac: right-click > Compress).
  3. Ask them to attach the new .zip file and send it again.
  4. When it arrives, save the .zip, right-click it and choose Extract All.
  5. If the .zip is also stripped, your mail server is scanning zip contents. Use the OneDrive link method instead.

2. Ask the sender to share a OneDrive or SharePoint link

  1. Ask the sender to upload the file to OneDrive, SharePoint, Google Drive or a similar service.
  2. In OneDrive they right-click the file, choose Share or Copy link, and paste the link into the email.
  3. Open the link in your browser and download the file directly.
  4. Your browser or antivirus may still warn about program files. That check is separate from Outlook's.

3. Have the file sent with a changed extension

  1. Only do this if you trust the sender and expected the file.
  2. Ask the sender to rename the file before attaching it, for example from setup.exe to setup.exe.txt or setup.doc.
  3. Save the attachment to a folder on your computer.
  4. In File Explorer, turn on View > Show > File name extensions (Windows 11) or View > File name extensions (Windows 10).
  5. Right-click the file, choose Rename, and restore the original extension (.exe). Confirm the change when Windows warns you.

4. Unblock a file type with the Level1Remove registry value

Classic Outlook for Windows only (Microsoft 365, 2016, 2019, 2021, 2024 all use 16.0). Does not affect new Outlook, Outlook on the web or Outlook for Mac. Ignored if your organization manages Outlook security by policy.

  1. Close Outlook.
  2. Press Windows+R, type regedit and press Enter. Back up the registry first with File > Export.
  3. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security (15.0 for Outlook 2013, 14.0 for Outlook 2010).
  4. Right-click in the right pane, choose New > String Value, and name it Level1Remove.
  5. Double-click it and enter the extensions to allow, separated by semicolons, for example .exe;.mdb
  6. Restart Outlook (restart the computer if it still shows the warning). The attachment now shows, but you must save it before opening.

5. Ask your IT or Exchange administrator

Work or school Microsoft 365 / Exchange accounts

  1. Send IT the sender's address, the file name and why you need it.
  2. For classic Outlook, admins can allow specific extensions through the Outlook Group Policy security settings (Remove file extensions blocked as Level 1).
  3. For Outlook on the web and new Outlook, admins control blocked types through the OWA mailbox policy (Set-OwaMailboxPolicy -BlockedFileTypes).
  4. The administrator can also release the file to you through a secure file share instead of changing policy.

6. Linked file attachments from a custom program

Classic Outlook for Windows, emails created by add-ins or internal systems

  1. Ask the developer to replace the attached-by-reference file with a normal hyperlink in the message body (Microsoft's recommended fix).
  2. If that is not possible, open regedit and go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security.
  3. Create a DWORD (32-bit) Value named AllowAttachByRef and set it to 1.
  4. Restart Outlook. This lowers security, so use it only on machines that need it.

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Still not fixed?

If the file is still blocked, check whether you are using classic Outlook, new Outlook or Outlook on the web, because registry changes only work in classic Outlook. On a work account, a company policy or mail filter is probably overriding your settings, so contact your IT help desk with the sender, subject and file name. If you did not expect the file, do not try to unblock it. Ask the sender through another channel whether they really sent it, since blocked attachments are a common way malware is spread.

Questions people ask

How do I open an attachment that Outlook blocked?

You cannot open it from that email. Ask the sender to resend it as a .zip file or a OneDrive link. In classic Outlook for Windows, you can also allow that file type with the Level1Remove registry value.

Why does Outlook block .exe and other files?

These file types can run code when opened, so they are a common way viruses spread. Outlook blocks them by extension, not because it found a virus in the file.

Can I delete a blocked attachment?

No. Outlook cannot open, save, delete or print Level 1 attachments. You can delete or archive the whole email instead.

Does Level1Remove work in new Outlook or on a Mac?

No. Level1Remove is a Windows registry setting that only affects classic Outlook. New Outlook and Outlook on the web follow the mailbox's server-side policy, which only an admin can change.

Why is my own sent attachment blocked in Sent Items?

The rule also applies to your own messages. If you attach a blocked file type, Outlook blocks access to it in Sent Items as well, and the recipient sees the same warning. Zip it or share a link instead.

Sources

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