OK. Got it! Thank you so much for your response. I am thinking of reset my macbook air to original factory setting because my laptop is running out of space. I do not have that minimal 10-15GB for it to run smoothly! ;-( There is so much going on in the storage, I already deleted the bulk of my music and photos (transferred to external hard drive) and computer is still not freed up a lot of storage!
How do I delete email messages in Mail? And when I do this, would it also deleted in my other apple devices like iphone or ipad?
I suppose I need to disassociate Mail usage, disconnect my email accounts to Mail app, since I have not been using it for a long time now, after setting it up. I usually check my emails directly via website.