Some popular methods for filtering and refusing spam include email filtering based on the content of the email, DNS-based blackhole lists (DNSBL), greylisting, spamtraps, enforcing technical requirements of email (SMTP), checksumming systems to detect bulk email, and by putting some sort of cost on the sender via a proof-of-work system or a micropayment. Each method has strengths and weaknesses and each is controversial because of its weaknesses. For example, one company's offer to " some spamtrap and honeypot addresses" from email lists defeats the ability for those methods to identify spammers.