It's very simple. You must have a log file that logs each visit. IP/Hostname/Country/Language/User-Agent/Email (add the email address encrypted in the URL you send, so you can detect the email address that is a spam trap). Then, you must check who owns the IP address. It's a tedious job, but if you build your own custom setup, you have high chances of hitting inbox in the future and avoid spam traps. Ideally is to filter the IPs, based on CIDRs. You build two lists of blacklisted IPs (datacenters, Tor, etc.) and whitelisted IPs (telecom companies, etc.).