We put SiteCanary in the user agent for that very reason -- we figure a really curious person could figure it out from there. That said, the fastest polling per

We put SiteCanary in the user agent for that very reason -- we figure a really curious person could figure it out from there. That said, the fastest polling period is 1 minute, so we think it would be pretty hard to DoS someone with this :-) Good idea with DomainKeys -- gmail seems to be alright with a reverse DNS lookup resolving to our domain. Cheers!