For some reason, I have an unused "unified mailbox" in my mailbox list.
I cant't delete it - it's greyed out.
I tried to move it away to personal folders, but it was only copied to there, and can't be deted from there either.

Hi,
that has happened to me before. But only because I took a cloud-based approach to Salesforce. Only after I deleted the connection and reinstalled it did everything run normally again. Check if there are other connections.

I was referring to my local KMail installation, sorry .....
KMail in a Kubuntu VM.
Mail provider is a GMX with a standard IMAP inbox account.