Unblock Attachments

Unblock Attachments buttonBelieve it or not but some people are actually creating and sending messages with attachments that will do your computer no good. Yes, you are correct; VIRUSES! There are a lot of file types in which a virus can reside. The most known are executables (programs) and scripts (automated processes).

A filename consists out of 2 parts; a name and an extension. The extension is the part of the filename after the dot. For instance the filename document.docx. Document is the name part and docx is the extension part. The last part decides how the file opens. In our example a docx file will open with Microsoft Word.

At the moment there is a limited (but not a definite) list of file extensions that are known to be potential viruses. Outlook takes no chances and blocks these files whether or not it contains a virus because Outlook is not a virus scanner and therefore cannot determine whether the file is safe or not. This is actually a good thing; even unknown viruses will be blocked this way! The downside of this is that occasionally you could receive a file that you know is clean but still can’t access. For instance; you receive a certificate, script or other program by e-mail.

Outlook blocked access to the following potentially unsafe attachments: click_me.exe.
Outlook informs you about any blocked attachments in the message header

Unblocking procedure

Registry Editor buttonBefore I tell you how you can still access those attachments, you must realize that you are creating a security risk on your computer this way. A good practice is to send these files in a compressed format (for instance zip or rar). This enables you to scan the file before unpacking and it will also take less inbox space and upload/download time! Another way to go would be to upload it to a cloud storage service like OneDrive or Dropbox and share it from there.

To unblock attachments involves changing the Registry. I strongly recommend to only edit the Registry when you actually receive a blocked attachment and when there is no other way to obtain that file again in a safer way (in a compressed format or via cloud storage) and you know for sure that the file is clean. Also, change back the Registry directly after you’ve saved the attachment. It probably sounds paranoid but you’ll be hitting yourself if you get infected because you “opened the door and invited the virus” yourself.

Alright, here goes;

  1. Make sure Outlook is closed.
  2. Open the Registry Editor;
    • Start-> type; regedit
  3. Locate the following key
    • Outlook 2007
    • Outlook 2010
    • Outlook 2013
    • Outlook 2016, Outlook 2019 and Microsoft 365
  4. Go to Edit-> New-> String Value and name the value Level1Remove (case sensitive!)
  5. Double-click on the newly created value and enter the extension including the “dot” that you want to open in Outlook. For instance .exe
    If you need to enter more than one extension you’ll have to type separate them by a semicolon like this; .exe;.bat;.url
  6. Press OK on the input box and close the registry editor
  7. When you open now Outlook, the attachments which hold those extensions aren’t blocked by Outlook anymore.

To let Outlook block those extensions again, follow the instructions again but instead of creating the Level1Remove value delete it.

If you are not comfortable with manually editing the registry or prefer easier access to block/unblock attachments you can also use OutlookTools (free).

Unblock attachments with OutlookTools
Easily unblock (and then block again) attachments with OutlookTools.

Change back the Registry settings directly after you’ve saved the attachment or you’ll leave a door open for infecting your computer with a virus!

List of blocked attachments

This is the list of file types that are blocked by Outlook (click here for the latest list published by Microsoft).

ExtensionFile type description
.adeAccess Project Extension (Microsoft)
.adpAccess Project (Microsoft)
.appExecutable Application
.aspActive Server Page
.aspxActive Server Page Extended
.asxASF Redirector file
.basBASIC Source Code
.batBatch Processing
.cerInternet Security Certificate File
.chmCompiled HTML Help
.cmdDOS CP/M Command File, Command File for Windows NT
.cntHelp file index
.cplWindows Control Panel Extension (Microsoft)
.crtCertificate File
.cshcsh Script
.derDER Encoded X509 Certificate File
.diagcabMicrosoft Support diagnostic tools
.exeExecutable File
.fxpFoxPro Compiled Source (Microsoft)
.gadgetWindows Vista gadget
.grpWindows program group file
.hlpWindows Help File
.hpjProject file used to create Windows Help File
.htaHypertext Application
.htcHTML component file
.infInformation or Setup File
.insIIS Internet Communications Settings (Microsoft)
.ispIIS Internet Service Provider Settings (Microsoft)
.itsInternet Document Set, Internet Translation
.jarJAVA archive file
.jnlpJava Network Launching Protocol
.jsJavaScript Source Code
.jseJScript Encoded Script File
.kshUNIX Shell Script
.lnkWindows Shortcut File
.madAccess Module Shortcut (Microsoft)
.mafAccess (Microsoft)
.magAccess Diagram Shortcut (Microsoft)
.mamAccess Macro Shortcut (Microsoft)
.maqAccess Query Shortcut (Microsoft)
.marAccess Report Shortcut (Microsoft)
.masAccess Stored Procedures (Microsoft)
.matAccess Table Shortcut (Microsoft)
.mauMedia Attachment Unit
.mavAccess View Shortcut (Microsoft)
.mawAccess Data Access Page (Microsoft)
.mcfManifest configuration file
.mdaAccess Add-in (Microsoft), MDA Access 2 Workgroup (Microsoft)
.mdbAccess Application (Microsoft), MDB Access Database (Microsoft)
.mdeAccess MDE Database File (Microsoft)
.mdtAccess Add-in Data (Microsoft)
.mdwAccess Workgroup Information (Microsoft)
.mdzAccess Wizard Template (Microsoft)
.mscMicrosoft Management Console Snap-in Control File (Microsoft)
.mshMicrosoft Shell
.msh1Microsoft Shell
.msh2Microsoft Shell
.mshxmlMicrosoft Shell
.msh1xmlMicrosoft Shell
.msh2xmlMicrosoft Shell
.msiWindows Installer File (Microsoft)
.mspWindows Installer Update
.mstWindows SDK Setup Transform Script
.msuWindows Update file
.opsOffice Profile Settings File
.osdApplication virtualized with Microsoft SoftGrid Sequencer
.pcdVisual Test (Microsoft)
.pifWindows Program Information File (Microsoft)
.plPerl script file
.plgDeveloper Studio Build Log
.prfWindows System File
.prgProgram File
.printerexportPrinter backup file
.ps1Windows PowerShell
.ps1xmlWindows PowerShell
.ps2Windows PowerShell
.ps2xmlWindows PowerShell
.psc1Windows PowerShell
.psc2Windows PowerShell
.psd1Windows PowerShell
.psdm1Windows PowerShell
.pstMS Exchange Address Book File, Outlook Personal Folder File (Microsoft)
.pyPython script
.pycPython script
.pyoPython script
.pywPython script
.pyzPython script
.pyzwPython script
.regRegistration Information/Key for W95/98, Registry Data File
.scfWindows Explorer Command
.scrWindows Screen Saver
.sctWindows Script Component, Foxpro Screen (Microsoft)
.shbWindows Shortcut into a Document
.shsShell Scrap Object File
.themeDesktop theme file settings
.tmpTemporary File/Folder
.urlInternet Location
.vbVBScript File or Any VisualBasic Source
.vbeVBScript Encoded Script File
.vbpVisual Basic project file
.vbsVBScript Script File, Visual Basic for Applications Script
.vhdVirtual Hard Disk
.vhdxVirtual Hard Disk Extended
.vsmacrosVisual Studio .NET Binary-based Macro Project (Microsoft)
.vswVisio Workspace File (Microsoft)
.webpnpInternet printing file
.websitePinned site shortcut from Internet Explorer
.wsWindows Script File
.wscWindows Script Component
.wsfWindows Script File
.wshWindows Script Host Settings File
.xbapXAML browser application
.xllExcel add-in
.xnkExchange Public Folder Shortcut