It is the individual file size not necessarily the whole folder that might be causing the problem.
As a test try setting the resolution on your camera to 640 x 480, take a picture and try emailing it.
Also, try sending something very small like a Word document which should send with no trouble at all.
Also, are you trying to send File Attachment or a Picture.
Try sending it as a file attachment.
Cameras these days are coming out with very large resolution but unfortunately the size of each image if taken at the best quality is murder on storage and uploading.
There is a
program that will make pictures smaller, is easy to use and is free if this is your problem.