Here ya go...

Ticket Refunds
Domestic Ticket

Valid for travel up to one year from the date of the original issue.
Refundable during the validity period only.
Once the ticket has expired, it is VOID and has no refundable value.
International Ticket

Valid for travel up to one year from the original date of issue.
Refundable for up to 2 years from the original date of issue.
Refund Procedures
For wholly or partially unused electronic tickets, refunds should be requested online via TravelNet.
For wholly or partially unused paper tickets, the tickets should be mailed directly to:
Delta Air Lines
Passenger Refunds - Dept 655
700 South Central Avenue
Hapeville, GA 30354

Helpful Hint: Make copies of all items being mailed.