Apparently it was sent to spam because it doesn't meet GMail's "Bulk
Sender Guidelines":
https://support.google.com/mail/answer/81126?hl=en-GB#authentication
tl;dr they want you using SPF, DMARC and DKIM. (The message headers
say spf=neutral, which I expect means nobody's set it up.)
These are very faffy (we just set this up at work) but worth it. But
also very faffy.
- d.