We've seen two reports of mail going to Gmail spam now, and this started happening to me, too.
I'm not sure how we can resolve this. Here's what we know:
- This wasn't happening until somewhat recently (1-2 weeks ago).
- DKIM and SPF headers haven't been changed in that time, and seem to be good (GMail adds headers implying it is happy with the SPF and DKIM settings).
- For me, Gmail displayed a banner with wording to the effect of "this is similar to other spam messages" to explain why it was marked, but I didn't get a screenshot of the exact text.
The only thing I can come up with offhand is that we are not setting "Precedence: bulk" on these messages, but should be.