Increase of False Positive in SPAM folder

[#1]

Over the past couple of weeks I've noticed that I'm getting a lot of false positives. They include circulars from Lastminute and other bone-fide mail lists that I'm signed up to.
Has anything changed in the filtering?

[#2]

I got one of my regulars put in the spam box yesterday

[#3]

Nothing's changed on our side, I'd recommend whitelisting the sender to avoid any chance of false positives in future.