When migrating over to EBS from SBS 2003 there are couple pre-migration steps that will save you a lot of trouble.
Change the IP address of your SBS LAN IP. This will allow you to point mail to it during the installation process. If you do not change it, then you will not be able to have mail hit it if one of your EBS servers (in our deployments, we’ve standardized on .1 for SEC, .2 for MGT, and .3 for MSG) are going to use x.x.x.2 (default SBS 2003 IP address fourth octet).
If running SBS 2003 Premium and using ISA or dual-NIC’d with RRAS firewall? Do yourself a big, big favor and separate out the firewall role (ISA or RRAS) from the SBS box. Using an inexpensive hardware device make it the default gateway BEFORE you start the EBS setup. The key here is to separate out your default gateway from the rest of the server roles on SBS that EBS is looking to so they are not all on the same IP address.