iCloud Mail issues

When I use mail on all my MacBooks it is constantly downloading messages and will never finish. This slows the machines down. I have had this logged with apple support for a couple of months and they say they have senior technicians working on it.

What is interesting is that if I try an log into my iCloud on any device even non apple and click on mail, it doesnt open and eventually shows an error message, so I suspect iCloud is the cause of all my issues.

Has anybody else had similar issues or does anybody have any solutions or ideas.

Apple support really have been hopeless. I was hoping the latest software release of Montgomery the other day was for me to fix this issue, sadly its made no difference!