Justice Chinembiri Bhunu said the e-mail purported to come from the laptop computer of the prosecution's star witness Michael Peter Hitschmann's can be used as evidence in senior Movement for Democratic Change politician Roy Bennett's sedition trial.
The prosecution alleges Bennett paid Hitschmann, an arms dealer, to buy weapons to assassinate Zimbabwe government officials.
Bennett is deputy Minister of Agriculture-designate but President Robert Mugabe has refused to swear him into office until he is cleared of the charges.
Justice Bhunu said the disputed emails which link Hitschmann to Bennett are relevant and vital to the determination of the case against Bennett. Hitschmann says he did not write the emails.
State counsel Attorney General Johannes Tomana hailed the ruling.
"We are saying that our link between Hitschmann and the accused person is in the communication between the two of them and those e-mails are the basis upon which the state finds the connection between Hitschmann and Bennett," he said.
Defense lawyer Beatrice Mtetwa argued the e-mails in question could have been written by anybody. In an attempt to show how easy it is to generate what she described as fake e-mails, she got a state witness to read an email, which turned out to purport to have been written by the prosecutor.
The prosecutor furiously argued that the cross examination was designed as a personal attack on him.
The defense lawyer Mtetwa argued she was merely trying to demonstrate the ease with which emails can be faked.
"We are trying to demonstrate that they are fake by placing before the judge emails that are in the names of people who know they did not make them and what better way to demonstrate that they are fake," she said.
The judge postponed the trial to Monday so he can decide on the acceptability of Mtetwa's cross-examination.