Pat, several things could delay the send of a large file, many times it is your AV program that has to test the whole thing before it will allow it to go out, Also, your ISP might not allow such a large files to be sent.
Either of these could be your problem.
Might want to break it up and send it in parts or delete the message and restart your outlook. Then see if you can just send the message without errors. It could be a bad email or as I said your AV checking the email.