I too am having Mac mail issues. Every time I attempt to create a new account the Mail APP crashes. I have my mobile me account set up, but it's take over 30-45 seconds to SEND a plain TEST text message with a TEST subject! I am so frustated I don't know what to do. To receive mobile me mail, through my MAC MAIL APP, it takes forever!
Everything worked great before I install Snow Leopard. I took my iMac into the Genious Bar, and they hooked it up and Poof no problems. So I bought a new $100.00 modem, as they said that was likely the problem. Nope. It's the MAC MAIL, I swear it is!
If I send the same text only message through my iPhone using my mobile me account, it's *snap* fast. I can receive to my mobile me account on my iPhone *snap* that fast.