Why would Exchange account work in Lion Mail and not Snow Leopard Mail? My wife's corporate e-mail is on an Exchange server. Everything — Mail, Address Book, and iCal — used to work fine on Snow Leopard. Then they changed some things, including the URL. Now Snow Leopard is having difficulties autodiscovering and setting up new accounts. If I'm persistent, Address Book and iCal will work. But Mail won't — it finds and tries to set up the account, seems to connect, even reads all of her mail folders on the server, but no existing messages appear. New messages received after the account is set up on her Mac will show up, but they will not be marked as read if read another way (Entourage/Outlook, webmail, etc.).

The weird thing is that all of this works seamlessly on Lion. If I set up her account through the Mail, Contacts, &amp; Calendars system pref pane, everything just works. Incidentally, it was using the settings in the Lion apps that allowed me to force Address Book and iCal on Snow Leopard to connect correctly. But I can't get Mail working no matter what I try.

Can anyone offer any suggestions? What would cause an Exchange server to talk to Mail on Lion correctly and not to Mail on Snow Leopard, even when I manually copy every possible setting?
SnowLeopard was the first version of OS X to have built-in support for Exchange 2007. Lion Mail officially added support for Exchange 2010 (according to the Lion Wikipedia page). If the Exchange server is running Exchange 2010, it's possible that contacts and calendars were compatible enough on Snow Leopard to just work, but Mail was not.