I open Mail and immediately there is a box with a message that says "Mail can't be sent, please select an outgoing mail server" or words to that effect. I have, in fact, specified a mail server.
Welcome to the all-new iMore Forums! - We're still moving some things around, so you may see a few quirks here and there, but we're working on getting things fully completed as soon as possible. For now, take a look around, and if you run into any major issues, please let us know in this thread!