Q: iMac Mail problem after downloading OS X El Capitan

I recently downloaded the OS X El Capitan update to my iMac since when I cannot SAVE Mail Preferences and have to do a six-point set-up Mail Preferences process each and every time I restart my computer.

Thai Mac technicians are unable to help. Their latest suggestion is to download the Mail program anew BUT the Apple technicians hesitate to do it because they don't know if my 70 mailboxes will be 'saved' or not! My livelihood depends on them! I have Back-Up but the back-up has the Mail Preferences malfunction. Can someone please help me?