According to The Verge, the developer behind MTS Mobile Mail, OJSC Mobile Telesystems, also worked on Samsung's email app and might have used the same identifier, com.seven.Z7, and the same signing certificate for both. If so, sharing these would lead to the installation on Samsung phones as the phone could have be tricked into thinking that it was an update to Samsung's native email app. Removing the app would be above the ability of the average user, but not for an Xda-developer forum poster whose solution you will find below. In the meantime, MTS Mobile Mail has been removed from Google Play.
source: MTSMobile via TheVerge
Some Samsung owners were surprised to find MTS Mobile Mail on their device (L), Removal instructions (R)