We have Exchange 2013 on premises environment. Users can send/ receive emails through Outlook client and OWA with attachment up to 20 MB. While users who are trying to send emails with attachment through iPhone - they can't do that.
Please note while we tested it on Android devices, there were no issues. It is only happening on iPhone devices.
While we were searching for this solution - we found out this link: https://technet.microsoft.com/en-us/library/hh529949(v=exchg.150).aspx
Which says, we need to make change in Webconfig file - i would like to know, if anyone have done this or is there any better solution available for this issue. please suggest.