This method usually prevents reception of an e-mail in the first place; thus, common anti-spam features like quarantine, redirect or delete can not be applied.
This method also distorts statistics of anti-spam programs because it is usually unknown how many mails (Spam or otherwise) would have been received through a certain connection.
Filtering by context includes methods like DNSBL lookups, domain or IP blacklists, etc.
For high volume mail sites, it is recommended to set up local copies of the DNSBLs and fill them through DNS zone transfers.
Some DNSBLs also allow only a certain amount of queries by day for every IP address to keep their expenses on bandwidth low.