Neither rain nor sleet.

Pict Via: locatis family

Via: phrases.org

"The original saying was actually "Neither snow, nor rain, nor heat, nor gloom of night stays these courageous couriers from the swift completion of their appointed rounds" and was said about 2500 years ago by the Greek historian, Herodotus. He said this adage during the war between the Greeks and Persians about 500 B.C. in reference to the Persian mounted postal couriers whom he observed and held in high esteem."