Do you perform a check with the username ?
This can lead to false positives (username are not unique across many forums) ...
Could you check by the webform only with the email's domain ?
Maybe this (these) domain(s) is (are) classified as spammer's domain(s).
I seem to be getting false positives, ie users trying to register are told they are spammers, I look at the log messages, but when I search for either their email or IP address - I do not seem to find a match on this site.
I am currently using version version 1.0 on my site, so will look to upgrade the plugin - but is this a likely solution ?
Thx
Lee