The Mail app on my Mac always shows a large number of drafts being saved, e.g. "Saving 2 of 647 drafts, 1.65GB". There are no drafts on the Mac to be saved. Apple say this is an issue only BT can correct. Can you advise please?
Have you logged onto your BT email account directly using a browser and checking to see if the draft folder has any emails stored there?
Yes. The drafts folder is empty.
How is your Mail app set up on your Mac. Is it an IMAP or POP3 account?
Does you Mail app draft folder have any items in it?
It would appear that the person you spoke to at Apple don't know their own product.
Have a look at these links and many more similar posts on the Internet.
https://discussions.apple.com/thread/8162326
https://smallbusiness.chron.com/turn-off-drafts-mac-mail-46742.html
I was not suggesting the links had a solution, more that because other people have had the problem that it would appear to be an Apple problem rather than a BTMail problem.