In certain scenarios or environments a client may be unable to send an email with an attachment from a mobile device. This usually occurs when the Edge Security Pack (ESP) is enabled with Client Certificates on the LoadMaster, but Client Certificates is also enabled on Microsoft IIS. In this scenario, Microsoft Exchange returns a 413 "Entity Too Large Error".
For further information, refer to the following Microsoft TechNet article:
Disable Client Certificates on IIS.
Navigate to IIS Manger > Exchange > Microsoft-Server-Active-Sync > SSL Settings > Client certificates = Ignore > Apply