Hi, welcome to justanswer.com! My name is Mike.
I will need some information before we begin:
Please go to the Apple logo at the upper left of your computer screen to About This Mac and give me this information:The exact system Version (listed right under OS X ??)ProcessorMemory
Are you there?
Open the Terminal app in Applications / Utilities copy and paste this line then hit Return:
Then restart and try Mail again.