Its probably this promotion for which I received an email:

Save 10% on your next flight when you purchase a qualifying ticket on nwa.com by September 1, 2005!

It’s easy to get your discount:
Purchase a ticket on nwa.com by September 1, 2005.
After your trip, we’ll e-mail you an E-Cert good for 10% off your next ticket. The E-Cert must be redeemed when purchasing a ticket on nwa.com by January 31, 2006 for travel through June 30, 2006.


Please retain this e-mail for your records.


Purchase Terms and Conditions:
Eligibility: This offer is not transferable or requestable and may only be used by the WorldPerks member to whom it is addressed.
Qualifying Activity: Purchase a ticket on nwa.com by September 1, 2005 for travel through January 15, 2006. Qualifying travel must be on a paid published fare booked in any class. Travel must be on Northwest operated flights 1-1999, Northwest Airlink flights 2700-2859, 2900-3699 operated by Mesaba and 2860-2899, 3700-3799, 4700-4799, 5600-5999 operated by Pinnacle Airlines.
Routing: Valid for travel within the U.S. 50. Travel may not originate or end in Minneapolis/St. Paul or Detroit.
Awards: 10% discount on one (1) paid ticket purchased on nwa.com by January 31, 2006 for travel through June 30, 2006. (Complete award terms and conditions listed below.) This award may be earned only one (1) time during the promotion.
Other: WorldPerks mileage credit will be awarded for travel awards earned in this promotion.
Fulfillment: The 10% discount E-Cert will be e-mailed to the WorldPerks member once qualifying travel has been completed. E-mail address will be the most current e-mail address in NWA's E-mail database. WorldPerks member must maintain a valid e-mail address with NWA E-mail from registration date until four (4) weeks after promotion ends to remain eligible for the award. If valid e-mail is not available to Northwest at time of offer fulfillment, no award will be given. Northwest Airlines is not responsible for any malfunctions associated with the e-mail collection process that result in incomplete information. Please allow 1-2 weeks after completion of qualifying travel for 10% discount E-Cert delivery.
Conditions apply: Additional restrictions may apply. Northwest is the final authority on the interpretation of these rules.
10% Discount E-Certificate
This offer entitles one (1) passenger a 10% discount off a qualifying published fare for roundtrip travel purchased on nwa.com. The discount is valid for travel within the U.S. 50. Travel is permitted on Northwest Airlines flights 1-1999 and Northwest Airlink flights 2700-2859, 2900-3699 operated by Mesaba and 2860-2899, 3700-3799, 4700-4799, 5600-5999 operated by Pinnacle Airlines.

Discount Travel E-Cert Terms and Conditions:
Ticketing/Travel Period:Ticket using the 10% discount E-Cert must be issued by January 31, 2006 for travel through June 30, 2006.
Reservations/Ticketing: Reservations must be made and ticket issued per the rules of the fare purchased. 10% discount E-Cert offer must be redeemed through nwa.com.
Booking Class: Per rules of the fare purchased.
Itinerary: Travel must be roundtrip. One-way travel/stopovers/circle trips/open segments/waitlisting is not permitted.
Routing: The 10% discount is valid for travel within the U.S. 50, except travel may not originate or terminate in Minneapolis/St. Paul or Detroit.
Transferability: This offer is non-transferable. Purchased or auctioned E-Certs are subject to confiscation and travel will not be permitted.
Changes/Refunds/Upgrades: Changes, cancellation, standby, upgrades, reissues are per the fare rule.
Combinability: Per the rule of the fare purchased. This E-Cert may not be combined with any other discount, promo fare also known as a web fare, infant/child fare, senior fare, certificate, E-Cert, ticket designator/reference code, bonus, upgrade (except WorldPerks® Elite card and Elite companion upgrades), WorldPerks Air Travel Award, or group fares.
Taxes: 10% discount does not include Passenger Facility Charges of up to $18 roundtrip, September 11th Security Fee of up to $10 roundtrip and U.S. segment tax of up to $3.20 per flown segment. A Segment is defined as a takeoff and landing.
E-Cert Offer: Limit of one discounted ticket per E-Cert. E-Cert has zero cash value and will be deemed fully used upon redemption and will not be honored retroactively or from participants in connection with the exchange of any wholly or partially unused ticket. Only original e-mails will be accepted. E-mail offer will not be replaced and cannot be requested. The use of the e-mail offer embodied herein by anyone other than NW in any advertisement for any purpose, or the purchase or the acquisition of certificates by persons or entities in the travel service business is strictly prohibited and will render the E-Cert offer void.
WorldPerks: WorldPerks mileage credit will be awarded for travel awards earned in this promotion.
Miscellaneous: Other conditions may apply. Offer is subject to change without notice. Northwest is the final authority on the interpretation of these rules.
Reference Code: PD001