I've been having this problem for almost a month now. I've been working with the Zimbra support folks in an attempt to get some insight as to why this problem started up all of a sudden.. I figured I might as well post what's happening in here too in hopes that someone else has had a similar problem.

A hand full of my users cannot send attachments through Outlook. They use the Zimbra Outlook Connector. If that same user logs into the webmail they can send attachments just fine. The maximum attachment size and maximum post size on my server has been cranked up to 800MB in an attempt to solve this issue. The attachment my users are trying to send are .pdf, .doc, and .xls files. None of them are over 8MB in size. We are running Zimbra version 4.5.11, the ZCO is version 4.5.11 as well (Filename ZimbraOlkConnector-4.5.11_GA_1749_4.5.505.msi). My clients are running Windows XP and Outlook 2003.

When I turn on debug mode for the clients Zimbra Connector client this is the error we are seeing when attachments are attemtping to be sent.
28-02-2008 14:55:33 [3296]: *107* DEBUG: Zimbra::Rpc::Connection::WriteToHttp, Byte written so far in current segment: 65536
28-02-2008 14:55:33 [3296]: *107* DEBUG: Zimbra::Rpc::Connection::WriteToHttp, Byte written so far in current segment: 131072
28-02-2008 14:55:34 [3296]: *107* DEBUG: Zimbra::Rpc::Connection::WriteToHttp, Byte written so far in current segment: 196608
28-02-2008 14:55:38 [3296]: *107* DEBUG: Zimbra::Rpc::Connection::WriteToHttp, Byte written so far in current segment: 262144
28-02-2008 14:56:38 [3296]: *107* ERROR: Zimbra::Rpc::Connection::WriteToHttp - failed on WinHttpWriteData(...), Error Code: 12002
28-02-2008 14:56:38 [3296]: *107* ERROR: Zimbra::Rpc::Connection::WriteToHttp - Total Bytes to write: 1048576
28-02-2008 14:56:38 [3296]: *107* ERROR: Zimbra::Rpc::Connection::WriteToHttp - Byte written so far: 262144
28-02-2008 14:56:38 [3296]: *107* ERROR: Zimbra::Rpc::Connection::WriteToHttp - Size of this segment: 65536
28-02-2008 14:56:38 [3296]: ZimbraXPLogon::SubmitMessage: RpcException: The request has timed out.
28-02-2008 14:56:38 [3296]: ERROR: Zimbra::Rpc::HandleConnectionFailure - RPC exception with error code: 12002 (The request has timed out.)
28-02-2008 14:56:38 [3296]: Zimbra::Store::ZimbraMessage::GetProps. this(1d38c90) inner(a8b5de4)
28-02-2008 14:56:38 [3296]: TRACING => Zimbra::Rpc::Connection::ReleaseProgressTask - _pProgressTask(0x1c35718)
28-02-2008 14:56:38 [3296]: Zimbra::Store::ZimbraMessage::GetProps. this(1d37240) inner(a876c84)
28-02-2008 14:56:38 [3296]: ZimbraXPLogon::SubmitMessage
28-02-2008 14:56:38 [3296]: IMessageWrapperImpl::GetRecipientTable
28-02-2008 14:56:38 [3296]: Zimbra::Store::ZimbraMessage::GetProps. this(1d37240) inner(a876c84)
28-02-2008 14:56:38 [3296]: Zimbra::Store::ZimbraMessage::SetProps. this(1d37240) inner(a876c84)
28-02-2008 14:56:38 [3296]: Zimbra::Store::ZimbraMessage::SaveChanges. this(1d37240) inner(a876c84)
On our server I am seeing error messages as well. The error below shows up in the mailbox.log file when my users try to send an attachment with the connector.
2008-02-22 14:27:54,807 INFO  [http-80-Processor72] [name=*edit*@*edit.com*;mid=136;ip=*edit*;ua=ZimbraConnectorForOutlook/4.5.505;] SendMsg - <SendMsg> LC(mbox=7d2794c4-e8ff-4a21-aa4a-112e3c292a6a)
2008-02-22 14:27:57,911 INFO  [http-80-Processor69] [] FileUploadServlet - File upload failed
org.apache.commons.fileupload.FileUploadException: Processing of multipart/form-data request failed. Connection reset
        at org.apache.commons.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:429)
        at com.zimbra.cs.service.FileUploadServlet.doPost(FileUploadServlet.java:346)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
        at com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:152)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:541)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
        at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
        at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
        at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
        at java.lang.Thread.run(Thread.java:619)
I've made registry changes to up the timeout value for sending messages. The default is 60 seconds. I change it to 600 seconds and still the same results.

HKEY_CURRENT_USER\Software\Zimbra\Connection\SendT imeout

Is where the registry changes were made.

After all of this testing was done and logs were sent I even went as for as reinstalling Outlook on my clients computers. When outlook was reinstalled it started sending attachments again, for about a day. The following day after outlook was reinstalled I have gone right back to the same problem. Any message with attachments will not send. It remains stuck in the outbox.

The folks as Zimbra support tell me the newest patch 5.0.3 will solve this problem for me, but that patch isn't out yet and its been nearly a month. The Zimbra folks said the patch was due out March 17th, but it still has not been released. I am at a loss. I need this to work now. Its been a month and my users are starting to grumble more and more. If anyone has had any similar experiences with this problem and have any insight into this I would be most grateful if you could help me with this situation.