As someone who is working with HHS in their innovation fellowship, I'd like to point out that HHS is very much moving to lower their email costs. I'm not certain the public information available, but much has happened since this information here was published: http://www.federalnewsradio.com/536/3384131/HHS-pacing-its-m...
Any high price for HHS mailboxes is related to the fact that there are several agencies with different rates (NIH, CDC, FDA, ACF, HRSA, etc are all part of the departmental umbrella of HHS) and some likely have different (and not as competitive) rates. My guess is that the rate quoted above is from one of the smaller agencies with a less competitively negotiated rate...but that's only an educated guess because if I knew the exact numbers I wouldn't comment on them publicly.
What's worth noting here though is that HHS is well aware of their costs, and if the procurement documents referenced above are any indication, then they are working to actively address this.