Exchange 2003 and 2007 (OWA Only)
Edit the web.config file on the CAS (Client Access Server). OWA uses ASP.NET and is influenced by the config of ASP.NET settings. The ASP.NET setting is "maxRequestLength". The "maxRequestLength" setting can be found in the web.config file. Configure the setting to the appropriate size amount needed to send and receive the desired size email.
Also review http://technet.microsoft.com/en-us/library/aa996835%28v=exchg.141%29.aspx
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