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Exchange troubleshooting: Allow large attachments for EWS clients such as Entourage and Mac Outlook 2011

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An e-mail message that is over the attachment size limit is moved to the "Drafts" folder. And the Entourage/Outlook 2011 client generates one of the below error codes.

  • Error -18596
  • Error -18493
  • Error: -18500

To resolve this issue, we need to follow below steps on the Exchange Client Access Servers in order to allow large attachments for EWS clients such as Entourage and Mac Outlook 2011:

Change "maxAllowedContentLength" parameter in "web.config" file:

From CMD change folder to "%windir%\system32\inetsrv\" and run below command:

appcmd set config "Default Web Site/ews" -section:requestFiltering -requestLimits.maxAllowedContentLength:<XX>

Notes

“XX” is the desired value in Bytes
In the event you are using a Small Business Server, replace "Default Web Site/ews" with "SBS Web Applications/EWS"

Change "maxRequestLength" parameter in "web.config" file:

Open "web.config" file located in the EWS Virtual directory "Microsoft\Exchange Server\ClientAccess\exchweb\EWS" will have the following entry

<httpRuntime maxRequestLength="13280" />

Note
By default, the max request size is approximately 13MB.

Change the item <httpRuntime maxRequestLength="13280" /> to <httpRuntime maxRequestLength="XX" />
**
Note**
Where “XX” is the desired value in Kilo Bytes.

Change "executionTimeout"  parameter in "web.config" file:

Open web.config file located in the EWS Virtual directory "Microsoft\Exchange Server\ClientAccess\exchweb\EWS"

Change the item <httpRuntime maxRequestLength="13280" /> to <httpRuntime executionTimeout="X" maxRequestLength="13280" />

Note
Where “X” is the desired time in seconds. Example value 600 for 10 minutes. Make sure the value is greater than the time it takes to upload the file. Change "connectionTimeout" settings in IIS.

Open "IIS Manager" > select "Default Web Site" > Click "Advanced Settings" > select "Connection Timeout" and change the value in Seconds (make sure the value is greater than the time it take to upload the file)

Make sure Transport settings and Individual users mailboxes supports above limit. Refer below articles for the same.

Entourage and Mac Outlook 2011 clients encode attachments in MIME.  This encoding increases the size of the attachment by about 33%.  Entourage and Mac Outlook 2011 show the size of the expanded attachment in the GUI.  This size further expands to additional 33% by Exchange server.

For this reason set the above limits (step 1 to 5) to about double the original attachments' file size.

Note
Increasing the allowed attachment size may cause performance issues in the Exchange environment. You have to consider the increase load a large attachment puts on the server infrastructure. You may experience one or more of the following symptoms if the attachment size is increased:

  • The Mailbox store database may become unstable.
  • High numbers of transaction logs are generated.
  • The average remote procedure call (RPC) latency spikes to more than 1,000 milliseconds.
  • Occasionally the average RPC latency spikes to more than 20,000 milliseconds.
  • CPU usage spikes from 25 percent to 50 percent. Occasionally, CPU usage nears 100 percent.
  • If a Microsoft Office Outlook client is also installed on the computer, you receive an RPC latency dialog box in Outlook. Additionally, Outlook does not work until the Microsoft Exchange Information Store service is restarted.

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