"phplist Spam blocked" error and I can't accept subscribe request from my friends

I received some email from phplist with title " phplist Spam blocked".

The email shows some of my friend’s email address but they couldn’t subscribe my newsletter.
I’ve added define(‘USE_SPAM_BLOCK’, 0); to myconfig.php, however I still cannot accept subscription.

How may my friends subscript without blocked??
Or, how may I disable this spam blocking function?