Outlook blocks some attachments because certain file types are commonly used to spread viruses and other threats. A block is usually a restriction on the file type—not proof that Outlook found malware in that specific file. For most people, the safest practical fix is to share the file through OneDrive or SharePoint and send an access-controlled link instead.
Why does Outlook block an attachment?
Email attachments are a common way computer viruses are transmitted, so Outlook blocks certain file types to reduce the risk to you and recipients. Microsoft’s examples include .exe, .bat, .vbs, and .js, as well as some database file types. These are examples, not a complete list; check Microsoft’s blocked-attachments list if you need to verify a particular extension.
The block alone does not mean Outlook scanned and identified your file as malicious. It means the file type is restricted under the applicable Outlook or organization policy.
What is the safest way to share a blocked file?
Microsoft recommends uploading the file to OneDrive or a secure network location such as SharePoint, then sending the recipient a sharing link. For OneDrive or OneDrive for Business, send the sharing link rather than using OneDrive to attach the file to the email.
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- Ask the sender to upload the file to OneDrive or SharePoint, or upload it yourself if you are the sender.
- Set the link’s permissions for the intended recipient. Confirm the recipient can access it without making the file more widely available than necessary.
- Send the link in the email. A link changes how the file is delivered; it does not establish that the file itself is trustworthy.
Microsoft’s instructions are available in its guidance on attaching files in Outlook. Access rules may depend on the sender’s organization and the recipient’s account.
Can you compress the file to get around the block?
Sometimes. Microsoft says a compression utility can place a blocked file in an archive—often a .zip file—with a different extension that Outlook may accept. Windows also includes a built-in option to create a compressed folder. However, acceptance of the archive is not a safety check: it can still contain the same risky file.
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Use compression only when you trust the sender and were expecting the file. Confirm an unexpected attachment with the sender through a reliable channel, be cautious with messages from unknown or untrusted people, and keep antivirus software up to date. See Microsoft’s attachment safety guidance.
Does the fix depend on your Outlook version or account?
Yes. Microsoft provides separate instructions for new Outlook, classic Outlook, and Outlook on the web, and its Windows troubleshooting pages ask users to choose the tab for the version they have. If you are following steps to attach, open, or save a file, first identify which Outlook version you use.
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If this is a work or school Exchange account, an administrator may control whether certain file types can be unblocked. Ask your organization’s IT administrator rather than assuming that changing a setting on your computer will override its policy. Microsoft’s blocked-attachments guidance explains the administrator role.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Should you rename the file or change Outlook settings?
Renaming an extension does not remove the file’s contents or make them safe, so it is not a sound routine workaround. Likewise, do not change security settings just to force a questionable attachment through.
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Microsoft documents an advanced registry procedure for some non-Exchange configurations, with guidance specific to Outlook 2010, 2013, and 2016. It is not a general first step for current Outlook versions. Prefer link sharing, or ask an administrator for help in a managed account. If considering a registry change, follow Microsoft’s version-specific instructions and make sure you understand the risks before proceeding.
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