Recently performed a migration from sbs 2003 to sbs 2008. very simple migration, no sql or fancy programs to worry about. just the domain, exchange and a few files. i followed all the steps outlined in http://technet.microsoft.com/en-us/library/cc546034(WS.10).aspx
to perform the migration. all seemed to go well until i turned the old server off....
it turns out that unless the old server is on, clinent machines cannot send or recieve emails.
all transport roles are setup correct from what i can see in the sbs 2008 box. and all email traffic is directed to the new sbs box via the router.
if i turn off the old server, cannot send or recieve anything, can connect to mailbox. but no transport.