Microsoft Outlook may automatically block access to specific attachments that are considered unsafe, such as executable files (.exe), script files (.js, .vbs), or registry entries (.reg). This security feature helps protect users from potentially harmful content that could compromise the system or the corporate environment. In this article, we will explain the reasons for blocking, provide methods to identify blocked attachments, and describe how administrators or advanced users can adjust Outlook settings.
Classification of attachments in Outlook
Outlook uses three levels to determine the safety of attachments:
1. Level 1 – Unsafe attachments
2. Level 2 – Medium-risk attachments
3. Level 3 – Other attachments
Level 1 includes file types that Outlook blocks from opening:
*.ade, *.adp, *.app, *.asp, *.bas, *.bat, *.cer, *.chm, *.cmd, *.cnt, *.com, *.cpl, *.crt, *.csh, *.der, *.exe, *.fxp, *.gadget, *.hlp, *.hpj, *.hta, *.inf, *.ins, *.isp, *.its, *.js, *.jse, *.ksh, *.lnk, *.mad, *.maf, *.mag, *.mam, *.maq, *.mar, *.mas, *.mat, *.mau, *.mav, *.maw, *.mda, *.mdb, *.mde, *.mdt, *.mdw, *.mdz, *.msc, *.msh, *.msh1, *.msh2, *.mshxml, *.msh1xml, *.msh2xml, *.msi, *.msp, *.mst, *.ops, *.osd, *.pcd, *.pif, *.plg, *.prf, *.prg, *.pst, *.reg, *.scf, *.scr, *.sct, *.shb, *.shs, *.ps1, *.ps1xml, *.ps2, *.ps2xml, *.psc1, *.psc2, *.tmp, *.url, *.vb, *.vbe, *.vbp, *.vbs, *.vsmacros, *.vsw, *.ws, *.wsc, *.wsf, *.wsh, *.xnk
Level 2 does not include any file types by default. However, an administrator can manually add certain file types to this category on a Microsoft Exchange Server if the mailbox is configured on such a server.
For example, if Outlook receives an email with a .reg file attachment (e.g., a file with new time zone settings for Windows XP), an information bar will display the following message:
Outlook blocked access to the following potentially unsafe attachments: TZ_WindowsXP.reg
Outlook interface features
The Outlook interface does not provide an option to open or view a blocked attachment. Typically, users encountering this behavior ask the sender to resend the file, either by changing its extension to a safer one or by archiving it into a zip or rar file.
Unblocking unsafe attachments through the Registry
While the Outlook unsafe attachment blocking system is justified and effectively prevents dangerous files from being launched, in some cases—such as for support services or software developers—this protection may be undesirable. In such scenarios, you can unblock and save the attachment by modifying the Registry.
Important: The blocking of “unsafe” file types in Outlook is a critical security feature. Disabling this mechanism may put your system at risk. Never open files from unknown senders and always scan received files with up-to-date antivirus software. It is recommended to receive files in archived form for greater safety. After completing the necessary tasks, be sure to restore the Registry settings to their original state.
Steps to unblock attachments:
1. Close Outlook
2. Open the Registry Editor and navigate to:
HKEY_CURRENT_USER -> Software -> Microsoft -> Office -> {Your Outlook version code} -> Outlook -> Security
Note: Each Outlook version has its own version code:
Outlook 2013 — 15.0
Outlook 2010 — 14.0
Outlook 2007 — 12.0
Outlook 2003 — 11.0
Outlook XP/2002 — 10.0
Outlook 2000 (SP3) — 9.0
For example, for Outlook 2010 the path will be:
HKEY_CURRENT_USER -> Software -> Microsoft -> Office -> 14.0 -> Outlook -> Security
3. Create a new parameter of type String Value named Level1Remove (case-sensitive).
4. Set the parameter value to a list of file extensions that should not be blocked by Outlook, separating them with semicolons. For example, to unblock files with the extensions .reg and .exe, specify:
.exe;.reg
This action will prevent Outlook from blocking the specified file types.
5. Close the Registry Editor and restart Outlook.
6. Now, when opening an email with a .reg file, Outlook will allow access to it.
Restoring default settings
To make Outlook block these attachments again, simply delete the Level1Remove parameter from the Registry.
