I duplicated that file, made a change to that setting, opened both the edited plist and the duplicate and BBEdit claims they are absolutely the same XML file, nothing different between them.
I think my minor troubles of the last few days stem from the preparations of opening iCloud. I have only one "Account" set up in Mail (probably set up automagically when I started using it).
[indent]Outgoing Mail Server (SMTP): Usually says .Mac (MobileMe)
Unfortunately, .Mac has been slow/flakey the last few days. Reseting that choice to "smtp.me.com:MYADDRESS" and then checking the Use only this server usually causes an immediate sending of the message. However, the reopening of the prefs window again shows ".Mac (MobileMe)". One would assume that these are actually the same server, just differently labeled to save confusing the user.
This is reinforced by the listing of only one server name in the list of servers, "smtp.me.com." If this assumption is true, it explains why there is no visible change in the plist file.
Niether the Activity Window nor the Connection Doctor show anything useful except that the incoming mail server sometimes takes quite a long time to connect. The smtp.me.com server is usually connected immediately. But the sending behavior would indicate just the opposite!
Of course, the whole Apple website seems bogged down just now...maybe their borrowing RIM's servers?