How can I send someone an attachment, if it's blocked by their copy of Outlook?

I'm going to assume that asking your recipient to work around the issue at their end is not an option. (It's outlined in this previous article: Why can't I open attachments in Outlook? Instead we're going to look at things you can do.

The simplest and most over looked option is simple: Does it have to be sent as an attached document at all? I've often received attached documents that are nothing more than lightly formatted text. It could just as easily have been sent as the body of the email message itself. Not only does this completely bypass Outlooks attachment filter but it makes for smaller email that is quickly sent and received.

Do you have the ability to send it in another format? If the document is only for the other person to read perhaps you can save it as a PDF which will preserve all of the graphics and formatting and is typically not blocked by default. If you don't already have the ability there are several Adobe PDF creation tools on the market such as Adobe Acrobat as well as others, some of which are free. In some cases even sending as a plain text file is sufficient though if that's the case I'd once again question whether it should be sent as an attachment at all.

Just ZIP it. The ZIP file format is a compressed archive. It's a way to compress and combine several files into a single file from which they can be later extracted. Windows XP actually has some rudimentary support for ZIP files built in so it's very likely that if you send your attachment as a ZIP file then your recipient will be able to access it. The popular ZIP programs include WinZip and PKZip.

Rename it. Outlook bases its decision on whether or not to block a file by looking at the file's extension (the characters after the last '.' in the file name). If you rename FOO.DOC to FOO.LEO then email it as an attachment then your recipient should be able to save FOO.LEO to disk, rename it to FOO.DOC, and be on their way.

Don't use email. It might just be that email is simply the wrong solution to transfer the file. The most common alternative if you have the ability is to upload the file to a web site and then send a link to the file rather than the file itself. Your recipient can then simply download the file at their convenience. Sending large attachments or sending attachments to a large number of people is considered bad "netiquette" anyway because of the resources and bandwidth used. This approach allows people to get their email quickly and then gives them a choice to download the file now or later or not at all.

Now it kind of goes without saying but I'll say it anyway - all of the techniques I've listed above are ways to circumvent Outlooks attempts to keep the recipient safe from viruses. If your document contains a virus then ZIPping it, renaming it, or uploading it won't make the virus go away when that document finally lands on your recipient's computer. This then implies that you as the sender have a responsibility to ensure that the documents you're sending are in fact virus free.