Since many large email providers use clusters of smtp servers, it's probably better to try to use SPF records or Domain Keys signing leaving the originating smt

Since many large email providers use clusters of smtp servers, it's probably better to try to use SPF records or Domain Keys signing leaving the originating smtp server as a latter resort. Even reverse dns lookups might be partly helpful but if someone controls the ip they could change that easily... Interesting problem over. I love the service though. Congrats guys!