Subject: **** thawte Production Maintenance Window May-17-2009 Notification ****
Why? Because Spam Assassin here is configured to add "***SPAM***" to the start of the Subject line when flagging suspected spam.
People have banner blindness and don't look at ads on web pages. Likewise, we're getting pretty good at recognising spam, especially when filtering software is flagging some of it for us.
Is there another way they could have drawn attention to the email?