I too often need to send clients large finished image files and really the best way to get the smallest file size is to create a pdf with all your fonts outlined etc so the final user doesnt have font issues etc.
The file will be large, I use some of my web space as an ftp, load the file on and then my client just downloads it. A 50MB file takes only minutes.
If your client cant do that for any reason, just use one of the many free sites to upload your file, a link is sent that all your client needs to do is click and the file downloads to them. At the end of the day the file your client receives needs to be correct and useable so I wouldnt go any other route really trying to send a file without outlines or in RGB etc just to be able to email. Just my opinion 