Our company just change the domain name, from 'aaa.com' to 'bbb.com'. I have added these 2 domain names on the exchange server. But, When my boss ask me to do some settings, when all of the users in the company received a email by 'xxx@aaa.com', the email server will repley a email automatically to the sender, said that 'we have changed our email domain to '@bbb.com'', etc... Does sb can kindly help me?

Exchange 2007 transport rules will do this. http://technet.microsoft.com/en-us/library/bb123927(EXCHG.80).aspx

Thanks, but my email system is Exchange 2K3.