Haven't messed around with it, but I have been exposed to SME a few years back when I did some volunteer work with an outfit promoted the use of open source for non-profit community organizations. SME was a good alternative to the complexity and cost of SBS a few years back for organizations with modest tech needs (e.g., using SAMBA F/P sharing in a GUI). I'm not sure that is the case anymore. (Native to) Linux email packages are notorious for having a steep learning curve, and I think you'd have to be very comfortable with bash/shell to get most tasks done. Someone did a post a few months back on alternatives to Exchange you could get in Windows or Linux flavors and that were free (unsupported), just can't find the forum thread at the moment.
Jim
James Adgate, CISSP IT Auditor and Compliance Specialist Data Loss Prevention (DLP) IT Security Policy and Risk Mitigation for Enterprises http://linkedin.com/in/jamesadgatech
In particular, the Kerio mail server looked interesting, it runs on Linux as well, I haven't used it but it appears to be a less expensive alternative to Exchange. What looks attractive is that you can get it bundled with integrated anti-spam/AV packages....
Although the SBS version of Exchange 2003 now allows for up to 75 gigs of mail storage, which should be more than enough for most small businesses..
Jim
James Adgate, CISSP IT Auditor and Compliance Specialist Data Loss Prevention (DLP) IT Security Policy and Risk Mitigation for Enterprises http://linkedin.com/in/jamesadgatech