Honestly, I would suggest sticking with Exchange. I know it's a bit pricey, but that would be easiest for everyone. Exchange 2007+ has really become a well oiled machine and I rarely have to deal with Exchange issues for my clients. Migrating from 2003 to 2010 is fairly straightforward and not overly complicated.
Feel free to pm me if you have any specific questions
If you are looking for a cheaper alternative and know a little bit of Linux, you could always use Mozilla Thunderbird and Zimbra which is something like 30/yr per user for the non-community edition (the community edition is free)
Thanks for the info. The only problem I have is we have out grown SBS 2003 and going to have to replace the server and start up a new exchange server. I just didn't know if there was something that was just as good or better for a cheaper price
For a cheaper price, not really - I mean, some people prefer using hosted Exchange, but when your environment is over 50 users, the benefits start to diminish and it becomes just as much work as hosting itself yourself would (in my opinion).
Also, a question that many people overlook - How many hours are you going to have to put in if you change solutions? Migration, user training, and the portability of any custom software that is in place.