PRB: Unable to Make More Than 256 Connections to Different Mailboxes Over a Short Period of Time

Using Web Distributed Authoring and Versioning (WebDAV) to open or access a large number of mailboxes in rapid succession may cause HTTP 500 errors to be returned on all mailbox accesses over 256.

All WebDAV requests to an Exchange server called under a single-user context are pooled under one session that is limited to 256 concurrent connections. This limit is due to the way that the session count is stored in memory. WebDAV exposes this limitation because it keeps connections open for 10 to 15 minutes.

To resolve this problem, obtain the September 2003 Exchange 2000 Server Post-Service Pack 3 (SP3) Rollup. For additional information, click the following article number to view the article in the Microsoft Knowledge Base: Alternatively, you can work around this problem in the following ways:

• Pause after each group of 256 connections, allowing the other sessions to timeout and new sessions to be created.
• If the code is running in a COM+ component as a specific user, adding additional server packages running under different user contexts will allow an additional 256 connections per package.
• Use another provider to access the Exchange mailboxes (for instance, ADO).

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section of this article.

Steps to Reproduce Behavior

1. Create 300 mail-enabled users with the alias naming convention of user(x), where x is the number 1 to 300.
2. Send an e-mail with the title "Test" to each of the 300 mailboxes.
3. Run the following code in an Active Server Pages (ASP) page:

   <% Response.Buffer = TRUE
   server.ScriptTimeout = 5000
   Dim Rec 
   Dim conn 
   Dim strURL 
   
      For mailboxloop = 1 To 300
         strURL = "http://servername/exchange/user" & mailboxloop & "/inbox"
         Response.Write strURL & "<br>"
         Response.Flush
         strURL = strURL & "/Test.EML"
         response.write "Open: " & strURL & "<br>"
         Response.Flush
         patchmail()
      Next
   
   Sub patchmail
    
       Dim intStartPos 
       Dim oRequest 
       Dim strTemp 
       Dim strXML 
   
       strXML = "<?xml version=""1.0""?>"
       strXML = strXML & "<D:propertyupdate xmlns:D='DAV:' xmlns:m='urn:schemas:httpmail:'>"
       strXML = strXML & "<D:set>"
       strXML = strXML & "<D:prop>"
       strXML = strXML & "<m:read>1</m:read>"
       strXML = strXML & "</D:prop>"
       strXML = strXML & "</D:set>"
       strXML = strXML & "</D:propertyupdate>"
   
       Set oRequest = Server.CreateObject("MSXML2.ServerXMLHTTP")
       With oRequest
           .Open "PROPPATCH", strURL, False
           .setRequestHeader "Content-type:", "text/xml"
           .Send (strXML)
       End With
       If (oRequest.Status <> 207) Then
           strTemp = "Error, status = " & oRequest.Status & " " & oRequest.statusText & "<BR>"
           Response.Write strTemp
   
       End If
       Set oRequest = Nothing
   Exit Sub
   End sub
   %>
   						

   The first 256 users open without problem, but from 257 on, you receive HTTP 500 errors.