And I'm having trouble trying to provide any answer! It would help a lot if you included at least some of:
What program and version are you using?
What have you done eg have you exported a video file from you project?
If so what file format and file size?
Or is 'Send as email...' an option within your program?
My first guess, as I've done it myself, is to simply have created a video clip too big for my e-mail program to accept. Solution -- in the export window pick a smaller size say 640x480 (not 1920x1080!), a slightly lower 'quality' setting, or even a different file type to get better compression eg MPEG4 (not uncompressed!). A further suggestion would be to check with your recipient what files types they are able to view, eg some PCs might not have quicktime or flash players installed.