From the DeltaNet:

Codeshare service on Mexican airlines suspended after rating change
July 31, 2010


The U.S. Federal Aviation Administration has changed the rating of Mexico’s civil aviation authority – the Dirección General de Aeronáutica Civil (DGAC) – from Category 1 to Category 2, prohibiting U.S. carriers from offering codeshare service on any Mexican airline. As required by this change, Delta will remove its code from Aeromexico flights.