The blocklist check will test a mail server IP address against over 100 DNS based email blocklists. (Commonly called Realtime blocklist, DNSBL or RBL).  If your mail server has been blocklisted, some email you send may not be delivered.  Email blocklists are a common way of reducing spam.  If you don't know your mail server's address, start with a MX Lookup.   Or, just send an email to ping@tools.mxtoolbox.com

Have more than 255 addresses?