Mail stuck moving messages
I have recently started to have a problem with the Mail app on my MacBook Pro with Cataline 10.15.6. It constantly says it is Moving Messages on the bottom left of the app. If new emails come through I can see them, read them, apparently delete them but although they go, when I log into my mail by another means like my iPhone the messages haven't moved. Every message I read or try to delete increases the number of messages that are moving.
I have quit, force quit, restarted etc to no avail.
Any ideas?
MacBook Pro 13″, macOS 10.15