I think that everyone is focusing on the wrong reason for the grabbing of these emails. This was not done for criminal investigation and Meuler doesn’t care if the courts rule that anything that results of this is the fruit of the poisonous tree. This was done for impeachment info which the Congress, Dems), can use for articles of Impeachment. They are looking for anything that can be the basis for an impeachment charge. Rules of evidence do not matter.
That is an interesting question, when in an impeachment against the President, the Chief Justice of the Supreme Court is the presiding judge and federal procedure and federal rules of evidence are approved by acts of Congress. The Federal Rules of Evidence are certainly binding on the presiding judge.