TL:DR? You can install an app (aka side load) on your Windows Phone running Windows 10 via email! Loving the little things they’ve done to help developers be more productive.
Several software partners have asked for this and very happy to report this works like a charm. I’m running 10.0.10512.1000 (latest public build available to Windows Insiders for Windows Phone) on a Lumia Icon. In order to get this to work, you’ll want to set your phone to Developer Mode and then send yourself the appxbundle. A tip from someone in the know…
“To get this to work…you’ve got to package the dependencies as well (e.g. VCLibs Debug or WinJS) that may not be installed on the device. Try building the appx for Release or send the appxbundle file instead (it should have the dependencies bundled).”
This so much better than Windows Phone 8.x side loading deployment!
Details on how this works here:
On a Windows Phone: You can install any app package (.appx) and run it without needing the certificate. Basically you are putting your phone in testing mode to install and try out an app that you trust. Do not install apps that you do not trust in this mode. You can install any .appx sent to you via email or on an SD card. Click the file to install if you have chosen developer mode for your settings.