System Preferences > Accessibility > Display
Check
- Reduce motion
- Increase contrast
Does this help?
Restart in Safe Boot mode and test Mail. Do the problems go away while booting in Safe Boot mode?
Safe boot
* shutdown
* start the Mac and *wait* for the startup chime to sound
* after the sound, press and hold down the shift key
* keep holding and *wait* until the grey Apple logo appears
* now release the shift key and just wait (it could take a few minutes)
* eventually the login screen appears with the words SAFE BOOT in the top right of the window
* continue to login to see if the problem goes away.
If the problems went away while in Safe Boot, download and run Etrecheck. Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here (and recommended by Apple Support personnel) in the ASC to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.
http://www.etresoft.com/etrecheck