Registration is at delta.com/thankyou. It's 75% bonus of redeemable miles (not MQM), so for Silver that already gets 25%, this adds up to a 'double'. I've been a long time plat with NW then DL but downgraded to a mere Silver in March. So I look at it as a re-engagement offer. To actually get the bonus, one has to fly 6 segments between registration and end of June. The fine print of the letter indicate that a 'direct' flight with a stop in between only counts at 1 segment. It's obviously targeted as I tried it with all the family members and not all of them were allowed to register. But the ones that were allowed didn't all receive the letter. So I'll say give it a try - you don't have anything to lose.