We recently switched to using an in house application for sending our campaign newsletters for one of our websites. It turned out that the new SMTP servers we were using weren’t trusted by email services such as Gmail, Hotmail, Yahoo, etc. as being valid sources of email messages. In come SPF, the Sender Policy Framework which these services use to determine if mail is coming from trusted domains for the sender. So, we had our hosting provider jack in some TXT DNS records for us that would allow our new SMTP servers to be trusted by SPF.
I figured all would be well and good. Email through these servers wouldn’t end up in a Gmail account’s spam folder. Not so. Apparently Gmail cache’s these records for a certain period of time, which I’m unaware of the length of. A nice post from Chris Linfoot gave me a little more insight into what might be going on. He mentioned in a my reply to his post that Google may indeed have not refreshed the cached record to my TXT SPF answer. Beyond that, Gmail may even consider my messages spam even if the messages pass SPF. Apparently Google doesn’t really use SPF messages to determine what really is spam and what isn’t. Interesting. My guess is they use it somehow in trying to figure out what is spam and what isn’t, but they don’t use it as the general rule of thumb for what should be spam. Its just one of their metrics.