Q: What has gone wrong with Apple Mail???

Mail is probably the most used application.

Apple Mail used to be superb.  Now it randomly stops working saying my log in details are incorrect.  Then it may start working again a day later without any changes.  This never happened before Yosemite.

What is wrong?  Is it a problem with Apple OS or iOS?  Is it a problem with my mail service provider? 

It has reached the point where I have had to revert to collecting mail via Internet explorer on my old Microsoft Windows laptop rather than my iMac.