Permission does not have an expiration data. Permission can be withdrawn at any time. Permission as granted can be a bit ambiguous, so if I ask my neighbor if I can use their garden hose to wash a window, it's not reasonable to think that the permission was to use the garden hose at any time for anything, unless he says that, but that result might be technically consistent with my request, just not a reasonable interpretation of the permission that I granted. In your proposed email usage, the intent is clearly to have an enduring relation, where a reasonable person would know that this was not a one-time permission. Jack has to explicitly withdraw permission.
Of course, permission can be withdrawn once the deed is done, but you're looking for a legal response stimulated by Sam's one-time bad behavior. Jack may be able to sue Sam for damages, because permission to use doesn't give you a right to trash a person's property. A reasonable person would know that permission to use an email account is not permission to vandalize a person's life.
Because this involves computers, you also get into the possibility of a "computer hacking" law, raising the specter of a criminal prosecution. There is such a law in the US, where it is a crime to "break into" a computer account and use the computer without permission. The courts do not interpret that law to mean "it's a crime to violate the terms of service of a website". Nobody can be criminally prosecuted for asking for specific legal advice on Law SE, even though it's against the rules.
There is also the possibility that the Sam's acts are intrinsically illegal, for example they might violate the defamation laws of the country, or the privacy laws ("exposing personal facts"). You'd have to be more specific about what Sam did.