Which Barracuda Spam Firewall to use?

I’m looking for an Antispam solution and stumbled upon Barracuda Spam Firewalls. My business is small (170 users) but with rather heavy e-mail usage. Now I’m trying to size the Antispam Firewalls from them and learned that the Model 2000 will support up to about 500 users and a load of 40.000 email/h.

Now the email throughput number seems a little bit high to me: Does anybody knows how those figures are meant/measured? Any recommendation on which model would fit a 170 user business?

Thanks!