I've run into this problem with other sites and found that the main factor is, as Donna said, the free E-mail address.
I had no idea that there were different "kinds" of addresses until I tried to register somewhere and the site very plainly said "NO FREE E-MAIL ADDRESSES"
Reasoning that the (unused up 'til then) E-mail account that came with my internet connection must be "UN-free" (I pay for it, right?), I activated that one and use it mostly for registration
purposes. I've had no problems since then.
I think the difference is that the latter is tied to my
IP address, which is pretty much the same as my phone number on your caller ID, since it's linked through my provider directly to my home.
Paid IP addresses are usually linked to such a bricks-and-mortar location, which for obvious reasons are not the base of choice for spammers
Lacking a "no free e-mail" firewall, sites like the Casita Club have to "hand verify", whatever that means, in order to rule you out as a potential bad guy.