I am having the exact same problem as Steve Kennedy a few posts back.
"When I press send I watch the fan for a couple of minutes before seeing the error message that invites me to select another server or try later. The outgoing mail server that shows up as being unusable is MobileMe (MobileMe). When I select smtp.me.com, I can send. Sometimes I can even send a couple of messages before the problem returns.
I'm using Lion and Mail 5.0 (1244.2/1244.3) on my iMac (mid-2010) and Verizon FIOS. Never had this problem with Snow Leopard or earlier. Don't have it with my MacPro running Snow Leopard on the home wireless network. Don't have it when using MobileMe mail on the Web, but that's a slow alternative."
I am getting REAL tired of the Lion quirks. <y desktop rearranges itself when it feels like it. Nothing consistent or repeatable. The folders just all appear on the right side and disregard how I have them organized on the desktop. I have set all of the recommended settings to no avail. A mind of it's own.
Help us Apple. Mail is almost unusable.