About Blocked Attachments in Outlook

Blocked attachments, also referred to as unsafe attachments, is a sometimes confusing technique that Microsoft added to Outlook to protect users.

Because so much malicious software (known as malware) has been delivered to user's computers in email attachments, Microsoft decided to have Outlook block many kinds of attachments. Remember that attachments are files. Every kind of file has something called an extension, a code at the end of the file name that identifies the type of file it is. For example, a document named "fred," that was created with Microsoft Word would likely have an extension of ".doc", which is short for document.

If you looked at the full name of this file on your computer, you would see it as "fred.doc". Outlook uses the extensions of each attachment to decide whether to block that attachment. Outlook 2003 blocks files with dozens of different types of files based on their extensions, to protect your computer.

Outlook 2007 does the same, blocking all the extensions that Outlook 2003 does, and perhaps a few more.

Note: Some time ago, Microsoft published an open letter (this link opens a new windows) to Outlook users explaining why they implemented the blocked attachments system despite the inconveniences and headaches it causes legitimate users trying to send files that Microsoft considers unsafe.

What to Do if You Want to Send a Blocked Attachment Type

If you try to use Outlook to send a message with an attachment type that it blocks, you see this message:

Warning about sending blocked attachment.

Outlook warns you if you are about to send an attachment it doesn't like.

You can send it, but you get fair warning that if the person who receives it uses Outlook as well, they may not be able to open the attachment.

So what should you do? As Microsoft explained in their letter, you can:

Somehow get your information into a file type that Outlook doesn't block. Beware though--just because Outlook doesn't block things like Word documents, it doesn't mean that they are safe. All it means is that Outlook doesn't block them and that youare responsible for ensuring that the documents contain no malware.

Rename the file to use an extension that Outlook doesn't block, then have the recipient rename it again after receiving it. Both you and the recipient need to be comfortable doing this. You can include instructions on how to do so in the body of your message to make it easier for them.

Use a zip program to package the file or files you want to send in a form that Outlook doesn't block. Then you can send instructions for how to unzip them in the body of the message, and hope the recipient has the tools to do the unzipping.

Post the files in a secure space on a network somewhere that both you and the recipient can reach. Again, you'll need to figure out how to do this (details vary from network to network) then include instruction telling the recipient how to retrieve the files.

Ask your corporate Exchange administrator to unblock the file type you want to send, at least temporarily.

As you can see, none of these options is user-friendly at all, but they are the only options available for sending blocked attachments.

What to Do if You Receive a Message With a Blocked Attachment

If someone sends you a message that contains a blocked (unsafe) attachment, you will see a banner like this in the header of the message:

Outlook will not let the attachment through. Period.

If you see this, you are out of luck. You cannot retrieve the attachment. If you want to receive the attachment, or at least the information in the attachment, the person sending it to you will have to use one of the techniques described under, "What to Do if You Want to Send a Blocked Attachment Type." Sorry.

Now can you see why blocking attachments caused such a furor when Microsoft first put this change into Outlook?

So exactly which types are blocked?

Do you want to find out exactly which file extensions Outlook is blocking? You can do so by searching Outlook help for the topic, "Attachment file types blocked by Outlook". That will give you a list of all the file extensions that Outlook is blocking on its own.

If you are using Outlook on a corporate network, the Microsoft Exchange administrators can make changes to the blocked attachment list. To find out what if anything they have changed, you will have to ask them.

From here you can:

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Return to the Seven Things You Should Know About An Email Attachment page.