The overall size of an email is restricted to 25Mb or less. How big is the attachment? Videos typically eat up a lot of space.
mp4 videos are very easily emailed for viewing by using a "cloud" service such as OneDrive. It takes a few minutes to get the knack of using this resource, but it's not too complicated. Another benefit for the recipients of your video is that when he/she clicks on the link you placed in your email, the video plays in full-screen mode, rather than the significanly smaller mode often used with an attached video file. And, the best benefit is that your email will be MUCH smaller than one with the video file attached.
My emails with video attachments never open the videos just a big black bar across the top
Try downloading the video to your computer and then play it.