Exchange 2013 OWA “The following files couldn't be attached” error

If are trying to add a large attachment to an email in Outlook Web App (OWA) you might get one of the following errors:
• The following files weren't attached because adding them would cause the message to exceed the maximum size limit of 10 MB: filename.
• The following files couldn't be attached: Filename. Please try again later.

As you can see from the first error, the problem is self-explanatory: the attachment is just too big. In Exchange 2013 the default maximum message size for an attachment is 10 megabytes (MB).

To Increase this limit you need to follow these steps:
  1. Increase the maximum message size for the organization by using the Set-TransportConfig cmdlet;
  2. Increase the maximum message size for the Send connectors by using the Set-SendConnector cmdlet;
  3. Increase the maximum message size for the Receive connectors using the Set-ReceiveConnector cmdlet;
  4. Increase the following settings in the OWA Web.config file:
    • maxAllowedContentLength (value in bytes);
    • maxReceivedMessageSize (value in bytes);
    • maxStringContentLength (value in bytes);
    • maxRequestLength (value in kilobytes).
  5. Increase the following settings in the EWS web.config file:
    • maxAllowedContentLength (value in bytes);
    • maxReceivedMessageSize (value in bytes).
  6. Stop and then restart the MSExchangeOWAAppPool application pool;
  7. Stop and then restart the MSExchangeServicesAppPool application pool.


IMPORTANT: due to the new architecture of Exchange 2013, steps 4 and 5 must be performed in both of the following locations:
  • The Client Access server on which the Web.config files are located in the following path: %ExchangeInstallPath%\FrontEnd\HttpProxy
  • The Mailbox server on which the Web.config files are located in the following path: %ExchangeInstallPath%\ClientAccess

Also, in step 4, please note that not all settings are present on both files.