That's USUALLY the case - that the mail server is/was down.
Comcast gets kinda pissy (from a consumer email end) with Mail checking every 5 minutes - every so often it rejects my password and makes me type it and all is well again. Every couple days, unless I shut down the mail client for an hour or so (which I rarely do - 2GB of RAM on a Leo box is plenty for me to leave everything I use running and in RAM so that it's instant when I click the Dock icon to get me a window...browsers, FTP client, MSWord - EVERYTHING that doesn't quit when you close the window).
"And and if you start to bleed, stop wiping." -whm1974