ada.dhs.org's name server at its ISP is currently (January 2009) down, and there is no ETA. Mail to ada.dhs.org is already starting to bounce.
And it looks like if you can still resolve the name ada.dhs.org at all then you are lucky. This really shows that secondary name servers being able to provide resiliency for DNS is a myth: with just ns.istop.com down (and ada.dhs.org itself still up), the ada.dhs.org domain is already disappearing from the surface of the Internet.
ada.dhs.org is currently (November 2007) under heavy DoS attack from spammers. Legitimate mail to ada.dhs.org may be slow.
I have friends that use ctm.net (Macau's telco monopoly), but they seem to be spammer-friendly: they have no abuse contacts listed in whois, none listed in their web site, and any mail to their generic mailbox just gets deleted without read. This is despite their claiming to not allow spamming.