Just finished a Migration from SBS 2003 sp1 to Server 2008R2 with Exchange 2010. (thanks demazter for the awesome guide)
during the migration email notifications were coming in just fine, after not so much.
just change the SMTP server name (servername not IP) to the new AD/Exch server.
authentication error after error.
ive also tried with IP address and FQDN nothing seems to work.