Earlier quoted context omitted.
If the FBI offers you immunity in exchange for implicating yourself in a crime, then they can't retroactively retract that offer after you've already confessed. This is true even in cases where the defendant was improperly offered immunity. And emails from an organization's domain name are generally legally binding. (Obviously this isn't legal advice.)
The FBI doesn’t offer immunity. The DOJ does. There’s also usually a signed document called a “proffer letter” or a “Queen for a Day” agreement that’s signed by an AUSA. I’m not sure an email would pass muster. Maybe it would, but it would certainly be a very big departure from the norm.
Fair. But if the FBI gave someone a cryptographically signed offer of immunity and the person then confessed, you don't think the case would get thrown out?