Rob, I know why activation e-mails going to spam folders (at least on gmail).
Response is too generic, same for everybody with exception of activation key.
If you add at least username or a little personalization (since you know the address (state, city, full name, etc.)), e-mails will not be filtered out by spam detectors.
It will take some time, because your mail server IP is most likely in black list of spamcop and/or others.
There some more issues:
– when I click on link in e-mail, it just shows page where I have to enter key (so key= parameter is lost or my account activated but response is wrong). I input key twice with same page displayed.
I tried to login and entered correct captcha and password. It errored out and presented slightly different page to login. I entered same password again and captcha and this time was successful.
And my 0.02c. WordPress is mostly platform for simple bloggers websites. You will have troubles with load since you have much more going on on EKN.