Mac constantly downloading emails

I have seen a thread about this but I do't use gmail so the solutions mentioned don't seem to apply. I use a BT internet server. I have looked at mailbox behaviours and draft seems to be set to save on Mac. I am not a techie so I need a simple explanation please! Last year I had email issues with time capsule too. Deleted all accounts and lost loads of stuff when senior Apple support 'helped' over 6 weeks. All trouble started with Sierra.I am currently using 10.12.6. Drives me crazy that 3000+ emails are constantly being downloaded.

Can anyone help?

Thank you