Hi
ActiveSync policy doesn't have an option to limit size of email sending and downloading, it is not practical to push the mailbox policies to mobile devices of difference brands and software versions in them since their design/functionality is different from
each others.
when the mobile clients try to send too large emails, Exchange CAS server will respond with error code 500 or 507 (based on the request details from different mobiles). The related setting exists in web.config file (C:\Program Files\Microsoft\Exchange Server\ClientAccess\Sync\web.config)
on Exchange CAS server which can be limited according to our needs
modify the 2 paramaters and enter the value you want to limit
<add key="MaxDocumentDataSize" value="type the value you need"></add>
<httpRuntime maxRequestLength="type the value you need"/>
Note: Ensure to take a backup of web.config file before you make above changes
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