Interesting feedback. I agree that it could be risky putting the word PayPal, Amazon or iPod or any other big brand name in the subject line and we're always very careful about this. However, just two weeks before we'd tested doing pretty much exactly the same thing in a previous email (ie we had PayPal in the subject line) and there had been no ill effect from MessageLabs or anyone else, so we felt it was safe to repeat two weeks later. Oh well. Live and learn!
PS given we sell flowers, having nice images of flowers in the email is very effective, far more so than a text email could be!