Some customers feel that letting people know what kind of SMTP server they are running is a security vulnerability. I can see their point. Once a vulnerability is in the wild all you need to do is find the servers... So some would like to change that banner to something that doesn't give away what the server is running. (Doing an EHLO and looking at the verbs may give it away though.) The following article gives detailed instructions on how to do this. This is not a recommendation to do this, but if you want to do it, it is possible.
281224 XCON: How to Modify the SMTP Banner
This works on Windows 2003 as well.