there is already quick time player in your device, every time you play a video on safari, let's say youtube video, you are using the inbuilt quick time player to play those videos. Same for whatever video you are streaming for iTunes whatever. Now if you receive a video by email or some how and you want to same it on your phone in other to play it, you will need to have an 3rd party application which will be capable to play quick time videos, the Evernote application, Download manager, VLC player, Stash file Pro, if any of these are install in your iPhone, then open up your email and locate the video file, touch and hold the video file until the dialogue box pop up asking you to chose a program to use to open the video with. Let's say that you have Stash file Pro installed in your iPhone Stash File which is i Know capable of playing quick time videos, once you touch hold the video in your mail box, and the dialogue box popup, you will see,
"open video with"
stash file pro
vlc
evernote
Others
select anyone of these program to play the video directly using the program you've selected, in the case of stash file Pro, it will automatically save the video into it's video folder for you, you can view it later at anytime, and if you do not want it there you can delete it, same thing to evernote, which i think also is capable of playing mp3 but i'm not sure about quick time video formats. As for VLC for iPhone it's capable of playing most of the video files out there, trouble is that i'm not sure if you will find VLC for iPhone nowadays in the apps store. Good luck.