I've been using this plug-in in my personal site for couple of years, to allow users to contact me via a contact form.
Today I've noticed that, when I try it from an incognito window (ie, as non logged-in user), the submission always ends up with a message like 'your IP has been flagged as SPAM'.
I've tried from multiple IPs (via sshuttle), I've tried the integration with Complianz (controls cookies and asks users to accept them), I've tried multiple browsers, I've tried whitelisting my IP in Zero Spam and Cerberos, nothing worked.
It was Zero Spam, it has a David Walsh method, but it doesn't work at all for me.