Have cleared all the messages in my inbox except 1 which I can't remove. Err mess says disc is full but I cant compact bec it says another process is operating

An old message is stuck in my inbox. No matter what I do it will not delete and I can no longer receive emails on this computer because I get this message: There is not enough disc space to download new messages. Try deleting old mail, emptying the Deleted folder, and compacting your mail folders, and then try again. I have cleared my delete and send files and substantially reduced my folders, but the same situation applies. I have tried to manually compact, but I then get the message that it cannot compact as another process is in operation. However, on my iphone or on another computer, I can still get updated emails.