I get this periodically, but on investigating the logs it is always mx.google.com (or one of its aliases) on the other end. I have been recommending that my domain users make sure our outgoing Smartermail server is in their SPF record.
Today I got it for our primary domain, which most definitely does have our outgoing server in its SPF record. See this lengthy log extract:
I cannot find our outgoing IP on any blacklists; our rDNS and EHLO banner match the dns, mail7.aleyant.net. What else could cause google to reject this? And in such an unfriendly manner -- saying "Go ahead" and then disconnecting when we do?