Why are email attachments zero-byte files on OWA and mobile devices?

Email attachments can sometimes arrive in Outlook Web Access (OWA) or on mobile devices connected to Exchange Server as zero-byte files. The file names are intact, and multiple files come through separately, but they're all simply reduced to files of zero length.

This strange behavior tends to express itself when several undesired conditions exist on the IIS server:

  • HTTP Static Compression is enabled on IIS. This causes any static files -- such as .HTM files -- to be compressed before being transmitted to the client for the sake of saving bandwidth on both ends.

  • An antivirus product is running on the IIS server.

  • The antivirus product is scanning the IIS compression directory, usually
    %systemroot%\IIS Temporary Compressed Files for IIS 6.0.

Antivirus packages have a method to exclude a given directory from scanning (it varies from program to program), so the easiest way to work around this issue is to exclude the aforementioned directory from scanning.

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Related OWA and mobile device administration resources:
Tip: Solving 'Subject: utd' email messages on Windows Mobile phones

Outlook Web Access Administration Guide

Learning Guide: Exchange Server mobile device management

If you have antivirus software running on the server in general, it's not required to scan this particular directory. (It may also be possible to fix this problem by updating to the most recent version of the antivirus software, which may be more "server-aware" and knows better than to scan that directory or tamper with the files in it.)

Also, make sure the compressed folders directory has not been changed. This would cause the antivirus product to scan it anyway.

In IIS Manager -> Web Sites -> Properties -> Service -> HTTP Compression, you can determine if compression is enabled and what directory it's set to use.

About the author: Serdar Yegulalp is editor of Windows Insight, a newsletter devoted to hints, tips, tricks, news and goodies for all flavors of Windows users.

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This was first published in June 2007

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