The issue relates to the fact that the video is being sent by email and the iPhone will compress the video for this purpose. It is not related to an email server capacity for attachments.
In order for video to be transferred at full resolution, the iPhone needs to be connected to a computer via USB. This will allow transfer of the entire file, for example using iPhoto.
When transferring a video file by email, a progress bar indicating compression is seen at the moment of transmittal, which measn the file is being compressed in resolution although it remains at full resolution within the phone until it is deleted.
In addition it is recommended to unlock the phone (if using a passcode) before connecting, to have access to this read/write functionality.