Although the post is about self-hosted mail, I wouldn't be surprised to learn something fairly major changed in Google's spam filter a couple of weeks ago.

We use Google Apps (i.e. Gmail) and two weeks ago one of my co-workers noticed that another co-worker's emails were going to spam. Even though we're sending/receiving within the one GApps domain.

Upon investigation I discovered that one co-workers emails were indeed going to spam for everyone, myself included unless he sent the email from his Windows machine. In which case it passed Google's spam filter no problem. His mail signature is more complicated on Apple Mail, 2 URL links (to our own domain) and an embedded image (NOT a URL).

He's been using that same signature structure for yonks; yet just in the last two weeks it's a no go.