Hi
I've seen the seem on Exchange Servers (5.5/200/2003) - but not always for all accounts on the exchange side.
Example: At one client with about 15 Users, it worked great for 13 Users, but not for two. One of those accounts had to be the boss of that company ;-(
It appears it is account specific.
Deleting and recreating the account on the windows side sometimes help.
Not in my case here though ;-(
I simply setup the exchane to recieve mails for alias@domainname.xxx AND for alias@pseudo.domainname.xxx.
I created all users in SME with an obscure password, and forwarded the mails to the stated address.
Made the necessary DNS entries so that Name Resolution wasn't an issue...
YMMV
Andy