There are a number of bugs in Apple Mail in High Sierra. Specifically:-
attachments stick in “downloading” mode - sometimes occurs when a message containing an attachment is moved by a mail rule to another mailbox
the rules used by smart mailboxes do not always accurately detect message status, flags or location.
Can anyone confirm whether or not these bugs have been fixed in Mojave? In Catalina?
I just saw this earlier today on Mojave.
This bug is well known, having been first reported several years ago. The problem arises if a) one has mail rules that move messages from the inbox to some other mailbox, and b) such a rule catches a message that contains an attachment. I have never yet seen any evidence (or discussion) that indicates the problem relates to the mail provider - this is a bug in Apple Mail, introduced initially by an update to (to the best of my recollection) El Capitan (or possibly Sierra)
Bother! (But thanks.)
Astonishing, isn’t it, that over a period of YEARS and across multiple OS releases, Apple are either unable - or can’t be bothered - to fix so serious a bug in a basic function of a mainstream app. …