1. New message comes in. I check it from Evolution. I delete it. It's gone from the server.
2. New message comes in. I check it from Evolution. I leave it. It stays on the server, but is marked as "not new". (Not sure of the implementation. Never thought about that detail before now.) Can Yahoo download only new items? Not sure.
3. New message comes in. I check from Yahoo. It is marked as read. In Evolution, I now see the header, but it is marked as read.
I never thought about the implementation for this. Is "mark read" something that depends on my client? Or is it an attribute in the IMAP message store?