Was staff instructed to remove Shillary's email mark on forwarded or replied to copies sent? If so, there is another problem for a lot of people.
Bottom Line Up Front: Any data transmitted via the Internet is copied, stored, and archived in multiple locations for a very long time.
To my knowledge, no one has published the source version (with the headers containing Internet Protocol (IP) addresses, domain names, transmit and receive times, and routing) of Hillary's emails.
Yet, given what has been published, the FBI and Intelligence Community (along with most of the other governments of the world) obviously have it.
Fundamentally, computers perform two functions; they add and they remember.
Most, if not all of the devices with the IP addresses and domain names listed in those email headers routinely made and permanently stored one or more backups of those emails as routing of IP data is probabilistic, not deterministic, there is no a priori means of determining the actual route of transmission, or if the data was successfully received, and there is a significant chance some or all of the data may have to be transmitted more than once.
This is a rather long-winded way of saying that those email headers are ultimately going to be the forensic provenance of Hillary's crimes, as well as the criminal activities of those she corresponded with.
Except for significant cases, the Intelligence Community is reluctant to prosecute espionage cases, because the legal proceedings often involve the compromise of their classified sources and methods.
In Hillary's case, as a direct result of her criminal (lack of) security activities, there is no need to use classified sources and methods, because the evidence is widely available from multiple commercial, open, unclassified sources. No evidence will be required that is not already widely compromised to this nation's enemies.
If the Justice Department refuses to prosecute, the document dump from this investigation will destroy the careers and reputations of a lot of people, and leave them wide open to civil lawsuits, with the heavy lifting of electronic discovery already done by the government for the plaintiffs' lawyers.
Under the Law of Untended Consequences, this is likely to cause the Democrats to become absolute zealots for tort reform at a speed and to an extent far beyond what any rock-ribbed conservative has dared to dream.