broadband customers. Since the vast majority of email that originates from these computers is spam, many mail servers also refuse email with missing or "generic" rDNS names. A Forward Confirmed reverse DNS verification can create a form of authentication that there is a valid relationship between the owner of a domain name and the owner of the network that has been given an IP address. While reliant on the DNS infrastructure, which has known vulnerabilities, this authentication is strong enough that it can be used for whitelisting purposes because spammers and phishers cannot usually bypass this verification when they use zombie computers to forge the domains. Rule-based filtering Content filtering techniques rely on the specification of lists of words or regular expressions disallowed in mail messages. Thus, if a site receives spam advertising "herbal Viagra", the administrator might place this phrase in the filter configuration. The mail server would then reject any message containing the phrase. Header filtering looks at the header of the email which contains information about the origin, destination and content of the message. Although spammers will often spoof fields in the header in order to hide their identity, or to try to make the email look more legitimate than it is many of these spoofing methods can be detected, and any violation of the standard on how the header is to be formed can also serve as a basis for rejecting the message. SMTP callback verification Since a large percentage of spam has forged and invalid sender ("from") addresses, some spam can be detected by checking that this "from" address is valid. A mail server can try to verify the sender address by making an SMTP connection back to the mail exchanger for the address, as if it were creating a bounce, but stopping just before any email is sent. Callback verification has various drawbacks: (1) Since nearly all spam has forged return addresses, nearly all callbacks are to innocent third party mail servers that are unrelated to the spam; (2) When the spammer uses a trap address as his sender's address. If the receiving MTA tries to make the callback using the trap address in a MAIL FROM command, the receiving MTA's IP address will be blacklisted; (3) Finally, the standard VRFY and EXPN commands used to verify an address have been so exploited by spammers that few mail administrators enable them, leaving the receiving