Screenshots of Windows 10 build 10051 have leaked onto the internet today, which don't reveal anything too interesting that differs from build 10049. It looks like the new Calendar and Mail apps are present, however the screenshots don't show them open.
Build 10049 was released to Insiders earlier this week, and included Project Spartan for Insiders to send feedback to Microsoft. Build 10051 seems to be continuing that trend of adding new universal apps, as the Outlook Mail and Outlook Calendar apps run across both PC and phone.
Windows 10 for phones build 12534 leaked via video the other day which revealed both new Outlook apps running. For those who don't know, 12534 is the same as 10051, meaning it makes sense to see the new universal apps included in this build.