Pasting clipped screen capture into osx email can't be seen by recipients
The default file type for screencaps in OS X is .png format. Perhaps your recipients' email readers cannot process .png files.

You can change the default file type by executing these commands in Terminal.

To change the default format to JPEG, for example, use these commands:

defaults write com.apple.screencapture type jpg
killall SystemUIServer


Other formats you can choose: gif, pdf and tiff.