Outlook will not allow some file types to be sent via attachments. You can learn about this annoying “Feature” of outlook at Microsoft’s official site at:

 

http://technet.microsoft.com/en-us/library/cc179163.aspx

http://technet.microsoft.com/en-us/library/cc178961.aspx

http://office.microsoft.com/en-us/outlook-help/blocked-attachments-the-outlook-feature-you-love-to-hate-HA001189421.aspx

http://go.microsoft.com/fwlink/?LinkId=188575

http://office.microsoft.com/en-us/outlook-help/blocked-attachments-in-outlook-HA010354417.aspx#_Attachment_file_types

 

As you can see Microsoft has written many articles about blocked or prohibited mail attachment file extensions. Below is a sample of some of the ones blocked.

 

File name extension    File type

.ade     Access Project Extension (Microsoft)

.adp     Access Project (Microsoft)

.app     Executable Application

.asp      Active Server Page

.bas      BASIC Source Code

.bat      Batch Processing

.cer      Internet Security Certificate File

.chm    Compiled HTML Help

.cmd    DOS CP/M Command File, Command File for Windows NT

.com    Command

.cpl      Windows Control Panel Extension (Microsoft)

.crt       Certificate File

.csh      csh Script

.der      DER Encoded X509 Certificate File

.exe      Executable File

.fxp      FoxPro Compiled Source (Microsoft)

.gadget            Windows Vista gadget

.hlp      Windows Help File

.hta      Hypertext Application

.inf       Information or Setup File

.ins       IIS Internet Communications Settings (Microsoft)

.isp       IIS Internet Service Provider Settings (Microsoft)

.its       Internet Document Set, Internet Translation

.js         JavaScript Source Code

.jse       JScript Encoded Script File

.ksh      UNIX Shell Script

.lnk      Windows Shortcut File

.mad    Access Module Shortcut (Microsoft)

.maf     Access (Microsoft)

.mag    Access Diagram Shortcut (Microsoft)

.mam   Access Macro Shortcut (Microsoft)

.maq    Access Query Shortcut (Microsoft)

.mar     Access Report Shortcut (Microsoft)

.mas     Access Stored Procedures (Microsoft)

.mat     Access Table Shortcut (Microsoft)

.mau    Media Attachment Unit

.mav    Access View Shortcut (Microsoft)

.maw   Access Data Access Page (Microsoft)

.mda    Access Add-in (Microsoft), MDA Access 2 Workgroup (Microsoft)

.mdb    Access Application (Microsoft), MDB Access Database (Microsoft)

.mde    Access MDE Database File (Microsoft)

.mdt     Access Add-in Data (Microsoft)

.mdw   Access Workgroup Information (Microsoft)

.mdz    Access Wizard Template (Microsoft)

.msc     Microsoft Management Console Snap-in Control File (Microsoft)

.msh     Microsoft Shell

.msh1   Microsoft Shell

.msh2   Microsoft Shell

.mshxml           Microsoft Shell

.msh1xml         Microsoft Shell

.msh2xml         Microsoft Shell

.msi      Windows Installer File (Microsoft)

.msp     Windows Installer Update

.mst     Windows SDK Setup Transform Script

.ops      Office Profile Settings File

.pcd     Visual Test (Microsoft)

.pif       Windows Program Information File (Microsoft)

.plg      Developer Studio Build Log

.prf      Windows System File

.prg      Program File

.pst      MS Exchange Address Book File, Outlook Personal Folder File (Microsoft)

.reg      Registration Information/Key for W95/98, Registry Data File

.scf      Windows Explorer Command

.scr       Windows Screen Saver

.sct       Windows Script Component, Foxpro Screen (Microsoft)

.shb      Windows Shortcut into a Document

.shs      Shell Scrap Object File

.ps1      Windows PowerShell

.ps1xml            Windows PowerShell

.ps2      Windows PowerShell

.ps2xml            Windows PowerShell

.psc1    Windows PowerShell

.psc2    Windows PowerShell

.tmp     Temporary File/Folder

.url       Internet Location

.vb       VBScript File or Any VisualBasic Source

.vbe     VBScript Encoded Script File

.vbs      VBScript Script File, Visual Basic for Applications Script

.vsmacros        Visual Studio .NET Binary-based Macro Project (Microsoft)

.vsw     Visio Workspace File (Microsoft)

.ws       Windows Script File

.wsc     Windows Script Component

.wsf     Windows Script File

.wsh     Windows Script Host Settings File

.xnk     Exchange Public Folder Shortcut