I've just moved from an ios phone to android on a p20 pro handset. I've set up my bt mail accounts, imap with SSL as advised by bt. I am recieving email no problem but if I've read an email on my PC it still shows as unread in my phone and visa versa, clearly the account is not syncing this element, it does sync if I delete an email on the PC it deletes on the phone. This seems to be a BT/Yahoo issue as I have a Gmail and Zoho account running quite happily and syncing just fine.
It worked ok on ios so perhaps this is an android issue, either way, your help would be appreciated.