Let's assume an email is an "ordinary email" for the sake of argument. This is a highly frustrating issue for me too, nearly every mail I send to gmail goes "poof" . . straight to spam.
My DNS report is clean.
My dedi and all of it's IP's are not on any RBL's, they too appear to be clean.
It's a simple plain text mail, like "Hello Bill, this is Bill, testing your gmail account" with a valid subject line.
Most of G's docs in relation to this tell you to mark it as "not spam" and G will "learn" to accept email from these actions.
This is not acceptable. I don't do bulk mail, and have emailed several clients with gmail accounts, all with the same result. "Oh here it is, in spam." ARGH . . . .
pageoneresults helped me work through some of the DNS issues, but even once those were resolved I still have this problem. And I still get this, like S.W.G. does:
"Received-SPF: neutral (google.com: 100.100.100.100 is neither permitted nor denied by best guess record for domain of firstname.lastname@example.org) client-ip=100.100.100.100;"
I **believe** this is because our dedicated server actually sends mail using the ISP's internal relay server. So although it comes from mydomain.com, the official sender is myisp.com. Does that make any sense?
If it's just this reason G is spam boxing my mail, I'm screwed. :-( This is not something we can reconfigure for our dedi.