The Secret Files of the Spy Dogs S01 E02 "Postal" or "The Secret File of the Contaminated Correspondence"

It all starts when Ralph suddenly notices a pattern in the mail that his owners are receiving: everything is a "You've won ten million dollars!" mailer. He also notices a strange slime on the envelopes, and sends the data off to the secret headquarters. He then receives word that the slime is from the evil Space