Everyone sending Clinton classified emails deliberately broke the same rule (not law) that she broke without knowing it. That’s why they didn’t mark their Secret info as Classified. But, none of them broke the law, because none had any criminal intent to damage national security.
The FBI’s first objective was to uncover classified emails on unclassified systems like Clinton’s personal email server (Report p.1 [A]). But they also found:
All classified emails on Clinton’s server “were drafted on UNCLASSIFIED systems.” (p.20 )
Many drafted on unclassified systems came the CIA, NSA, etc., and even the FBI. (p.21 [C])
No emails identified by the FBI as “classified when sent” were marked Classified. (pp.2 & 20)
Clinton contributed to only 11 of the 81 classified email chains. (p.22 [D]) (And these “contributions” appear to be only unclassified questions and comments.)
Very interesting details that link back to the FBI reports.
“Authors of the e-mails stated that they used their best judgment in drafting the messages and that it was common practice at State to carefully word e-mails on UNCLASSIFIED networks so as to avoid sensitive details or “talk around” classified information.” (FBI Report page 24)
Apparently this was also common practice at the CIA, NSA, FBI etc. One official “stated the right method of communication was whichever method allowed for the fastest possible dissemination of the message.” (FBI Report p. 23)
One official told the FBI (p. 23), the “information he received from other USG agencies was ‘technically probably classified’ but that ‘you can’t do business that way.”
This is why once every two or three weeks, someone sent Clinton classified information from their unclassified server and didn’t mark it as classified. Usually, they tried to disguise the classified info. With all these deceptive signals from experienced experts, you can see why Clinton did not realized that 0.6% of her emails should have been marked classified.