Espionage is the crime of spying on the federal government for the purpose of transferring state secrets to a foreign country. 18 U.S. Code § 793 sets out the specific acts and allows for the prosecution of anyone who "for the purpose of obtaining information respecting the national defense with intent or reason to believe that the information is to be used to the injury of the United States, or to the advantage of any foreign nation..." collects or attempts to collect national defense information. The prosecution of an espionage case requires proving that (1) the information transmitted is classified government information or relates to national defense; (2) the defendant acted with the intent or reason to believe the information will harm the United States or help a foreign nation (not necessarily an "enemy" of the United States); and (3) there was either a willful communication, transfer, or receipt of the information or an overt act in furtherance of a conspiracy to commit espionage.
The material hacked from DNC servers was not classified defense information. If anybody has been giving state secrets to the Russians that's espionage; but it's not espionage to try to sabotage an opponent's political campaign. I wouldn't be surprised if Trump had done that but not during the campaign; he didn't have any state secrets until after he was elected and briefed by the IC.
Espionage is a very specific crime. I doubt that much of what Mueller is uncovering would be considered espionage because most of it occurred before the election and the suspects did not have access to national defense secrets. However, there are many, many other crimes being investigated, most of which probably involve money laundering as well as computer hacking.
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