Client Certs - ActiveSync Attachments Fail
In certain scenarios when using a mobile client to send an email with an attachment, the server might reject this email and return HTTP Error Code 413, which translates to "Entity too Large". This can occur if you are using ESP with Client Certificates, but also have client certificates enabled on your IIS Server.
Disable Client Certificates on IIS Server
Navigate to IIS Manager > Default Web Site > Microsoft-Server-ActvieSync > SSL Settings > Client Certificates = Ignore