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Unblocking attachments in Outlook

How can you get an email attachment that Outlook for Windows has blocked?

Outlook will automatically block email attachments with certain extensions.  We’ve copied the full list of blocked extension below.  Developers and IT people are the most likely to get frustrated by Outlook file blocks.

In many cases the blocked file is really plain text code or script which could be resent within an email message body instead of an attachment.  The receiver just selects part of the message and makes a new file.

Here’s some of the file types that you might want to get past Outlook with some comments on alternative ways to send the file.

.asp   an Active Server Page, commonly used on web sites.  A plain text file.The more recent .aspx equivalent is not blocked.

.cer, crt Internet certificate.  These are usually delivered by SSL providers via a .ZIP file.

.cmd  or .bat    Windows /DOS command file.  These are plain text and usually short so they can be sent in a message body and easily recreated.

.js  Javascript source code. Again, this is plain text and could be send in a message body instead.

.pl  Perl script.  Another plain text file.

.mdb .mda and other MS Access files.  Use ZIP instead (most compress into a much smaller file).

.ps1 and other PowerShell scripts.  Use plain text instead.

.reg  – more plain text

.url a Internet link, put in a message body instead.

Most of these are blocked for good reason.  Under no circumstances should you open a .exe attachment even from someone you trust.

But sometimes you’ll get an email with attachment that you need.  How to bypass the Outlook block?

The easiest option is to ask the sender to resend the files in a different format (like ZIP or RAR), to a different email account or via an online file share. But assuming those options aren’t available.

WARNING:  use any attachment you get after unblocking with extreme caution.  Microsoft hasn’t blocked these attachments to be annoying, they are trying to protect their customers.

Make sure that the email attachment comes from the genuine sender and it’s not a spoofed email that the sender has no clue about.

You can’t direct change what Microsoft calls the ‘Level 1’ block list for Outlook attachments but you can manually exclude an extension from the current list.

The Outlook attachment blocks can be removed in the Registry at:

HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook\Security

The <version> of Outlook is

  • Outlook 2016 16.0
  • Outlook 2013 15.0
  • Outlook 2010 14.0
  • Outlook 2007 12.0
  • Outlook 2003 11.0
  • Outlook 2002 (XP) 10.0
  • Outlook 2000 9.0

For Outlook 2016 it’s

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security

Now add a String value  Level1Remove 

The data is a list of extensions you want UNblocked with a dot/period and a semi-colon to separate.

To Unblock just Access .MDB files  use   .mdb 

To Unblock .mdb and .bat files use  .mdb;.bat

After a registry change, restart Outlook and the blocked attachment is now available.

If you’ve made the registry change just to access a single attachment.  Save the attachment to your computer (and check it for nasties) then remove the registry entry.

If you need to unblock occasionally, rename the key to disable it. That makes it easy to quickly restore in the future.

Outlook will ignore a registry entry with a name it doesn’t recognize.

Here’s a list of blocked attachments, as advised by Microsoft.  There’s a search field at the top (depending on your browser).

File extensionFile type
.ade Microsoft Access project extension
.adp Microsoft Access project
.app Application file
.asp Active server page
.bas Microsoft Visual Basic class module
.bat Batch file
.cer Internet Security Certificate file
.chm Compiled HTML Help file
.cmd DOS CP/M command file, Microsoft Windows NT command script
.cnt Windows Help contents file
.com Microsoft MS_DOS program
.cpl Control Panel extension
.crt Security certificate
.csh Script file
.der DER Encoded X509 certificate file
.exe Executable file
.fxp Microsoft Visual FoxPro compiled program
.gadget Windows Gadget
.grp Windows program group file
.hlp Help file
.hpj Help project file
.hta HTML program
.inf Information or setup file
.ins Internet Naming Service
.isp Internet Communication settings
.its Internet Document Set file
.jar JAVA archive file
.js JavaScript source code
.jse JScript Encoded script file
.ksh UNIX Shell script file
.lnk Windows shortcut
.mad Microsoft Access module shortcut
.maf Microsoft Access shortcut
.mag Microsoft Access diagram shortcut
.mam Microsoft Access macro shortcut
.maq Microsoft Access query shortcut
.mar Microsoft Access report shortcut
.mas Microsoft Access stored procedures
.mat Microsoft Access table shortcut
.mau Media attachment unit
.mav Microsoft Access view shortcut
.maw Microsoft Access data access page
.mcf Manifest configuration file
.mda Microsoft Access add-in program
.mdb Microsoft Access program
.mde Microsoft Access MDE database file
.mdt Microsoft Access data file
.mdw Microsoft Access workgroup
.mdz Microsoft Access wizard program
.msc Microsoft Common Console document
.msh Windows PowerShell script helper
.msh1 Windows PowerShell script helper
.msh2 Windows PowerShell script helper
.mshxml Windows PowerShell script helper
.msh1xml Windows PowerShell script helper
.msh2xml Windows PowerShell script helper
.msi Windows Installer package
.msp Windows Installer update package file
.mst Visual Test source file
.ops Microsoft Office profile settings file
.osd Open software description file
.pcd Microsoft Visual Test compiled script
.pif Windows program information file
.pl Perl script file
.plg Developer Studio build log
.prf Outlook profile file
.prg Program source file
.ps1 Windows PowerShell
.ps1xml Windows PowerShell
.ps2 Windows PowerShell
.ps2xml Windows PowerShell
.psc1 Windows PowerShell
.psc2 Windows PowerShell
.pst Microsoft Outlook personal folder file
.reg Registration entries, Registry data file
.scf Windows Explorer command file
.scr Windows screen saver
.sct Windows script component
.shb Windows shortcut to a document
.shs Shell Scrap object file
.tmp Temporary file or folder
.url Uniform Resource Locator (Internet shortcut)
.vb Microsoft Visual Basic Scripting Edition (VBScript) file
.vbe VBScript Encoded script file
.vbp Visual Basic project file
.vbs VBScript script file, Visual Basic for Applications script file
.vsmacros Microsoft Visual Studio .NET binary-based macro project file
.vsw Microsoft Visio workspace file
.ws Windows script file
.wsc Windows script component
.wsf Windows script file
.wsh Windows Script Host settings file
.xbap XAML browser application
.xnk Microsoft Exchange public folder shortcut

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