Approach to Troubleshoot Outlook 2011 Mac HTTP Error 18500

Though the integration of Outlook for Mac 2011 with Exchange Server could lead to superior features and enhanced adaptability, it also has some drawbacks associated with Exchange Server Mac. Users often encounter a common error delivery code 18500 while sending any form of email with or without attachments. The blog will be discussing about the causes and solution to troubleshoot Outlook 2011 Mac http error 18500.

Possible Causes of the Outlook 2011 Mac http Error Code 18500

This Error comes with the following message that states, “Server cannot fulfill request” when using Outlook for Mac with Exchange Server.

outlook 2011 error code 18500 - http error


The main reason for the occurrence of 18500 Error is when the Outlook user sends files attachments of huge size. The files above 1 MB are often considered as large files and error was seen while sending such files. It means that there is a problem with the authentication and the EWS directory.

Manual Approach to troubleshoot the Outlook 2011 Mac http Error 18500

If the Outlook for Mac 2011 user gets a message about the failure of delivery of attachments file, it is clear that there is an issue with EWS directory and validation. To overcome this error, we will make some modifications in EWS folder under IIS (Internet Information Service) following the steps below:

  • Firstly we need to open IIS and click on EWS folder
  • Double-click on it to open it and go to the Authentication section
  • Select the Basic Authentication settings and enable it.

outlook 2011 error code 18500 - http error

Error should be resolved by the above procedure. However, if you have very large file size i.e., above 13 MB, that needs to be sent, error will appear. In this case we need to make modifications on the web.config file. The web.config file is located under the EWS virtual directory Microsoft\Exchange server\ClientAccess\exchweb\EWS that has an entry, which is normally set to approximately 13 MB.

outlook 2011 error code 18500 - http error

To find a solution for the error, we will modify “maxRequestLength” parameter in web.config file by changing the item:

<httpRuntimemaxRequestLenth=”13280”/ > to < httpRuntimemaxRequestLength = ”KK” >
Where ‘kk’ is kilobytes.

Conclusion: The blog has come into picture to discuss about the Outlook 2011 Mac http error 18500 that user encounters while using attaching Mac Outlook 2011 with Exchange Server. The issue arises when the server restricts while user tries to send large-sized attachment files displaying error 18500. The Basic authentication settings can be enabled that will allow user to send files above 1 MB whereas for sending files above 13 MB, parameters in the web.config file has be modified. However, due to high risk involved in making changes at this file, it is not a right approach.

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