Between October 1 and November 15, Delta is offering a 15% bonus on transfers from American Express Membership rewards of 75,000 points or less and a 20% bonus on transfers of more than 75,000 points. Registration required.
Note that the fine print of the promotion says that pre-registration begins September 20th, though the website is taking registrations now. If you intend to use the promotion I’d go back and re-register in a couple of days.
The fine print also says the offer begins October 1, even though the large print says it begins “now”. This is really annoying, but just be aware that you should wait until October 1 to do any transfers to Delta from Membership Rewards.