Moody's lowered its outlook on FedEx Corp. to negative from stable, saying the aggressive financial policy of the delivery services giant could prevent it from meeting credit metrics that are in line with its current rating.

The rating agency said FedEx would be "hard-pressed" to achieve debt levels and credit metrics that are more supportive of its Baa2 rating if it continues to repurchase shares and fails to significantly reduce capital investment.

FedEx's stock buybacks have historically exceeded its free cash flow, according to Moody's, leading to an increase in funded debt and leaving little credit metrics cushion for periods of weaker demand and earnings.

Moody's said FedEx's consolidated margins will remain under pressure at least through fiscal 2021, due to weakening economic growth and the slower-than-expected integration of its TNT business.

Moody's affirmed FedEx's senior unsecured rating at Baa2 and its short-term rating at P-2.