Exchange Server ActiveSync uses certain custom extensions on the HTTP protocol -- called HTTP "verb" commands -- that are sent in the context of an HTTP request to tell the server what the mobile client
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is trying to accomplish.
Most of us are familiar with the more commonly used HTTP verbs GET, POST and HEAD, which make up the vast majority of requests. But many people are unfamiliar with the custom HTTP verbs that SharePoint (and a number of other Microsoft products) use.
One such verb is X-MS-ENUMATTS -- meaning "enumerate attachments," with the X-MS- being a way to designate the verb as a custom Microsoft creation. (It's considered good Internet form to prefix a custom header of any kind with an X- as a way of distinguishing it from RFC-standard headers.)
The X-MS-ENUMATTS HTTP verb is used to enumerate all attachments in a given message. If it's blocked by a proxy, firewall or server-side constraint, any attempts to synchronize attachments to a mobile device via Exchange ActiveSync will fail with a cryptic error.
The same issue also appears in Windows SharePoint Services or SharePoint Portal Server if you configure a document library to allow e-mail insertion, but have X-MS-ENUMATTS blocked. The SharePoint server will throw a "503 Service Unavailable" error.
X-MS-ENUMATTS commonly winds up being locked down on the server side when Microsoft's UrlScan security tool is used, because it restricts the types of verbs that can be used with Internet Information Server (IIS).
If you place too much restriction on the verbs that IIS can handle, Exchange ActiveSync and other functions might break, so too much restriction can be as bad as too little.
Microsoft Knowledge Base article 823175, Fine-tuning and known issues when you use the UrlScan utility in an Exchange 2003 environment explains how to avoid issues when using UrlScan in conjunction with other applications.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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This was first published in July 2006
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