Via this Milepoint thread, Delta’s offer of a 50% bonus on transfers from American Express Membership Rewards for first time transfers is back through September 30. Transfers of 100,000 miles or more will generate 25,000 elite qualifying (status, not redeemable) miles as well.
Registration is required, and the terms and conditions explicitly say that the bonus cannot be combined with other offers. They apparently learned their lesson after the recent error that had them awarding 80% bonuses…
Presumably it won’t stack with the current (up to) 40% bonus on transfers from Amex to Delta. But folks who don’t qualify for the 50% offer, because they’ve transferred points from Membership Rewards to Delta Skymiles in the past, can use this one instead.