Originally Posted by
brunos
It is not "some of your miles" that will expire, it is all of them if you don't take a qualifying ST flight before the deadline.
Not completely true. When -for example- your last flight was about 20 months ago, all miles earned up to then will expire. If you collected miles a few months ago for a car hire, hotel stay or whatever other source, these miles stay valid for 20 months from that date.