These letters must be original. They do not accept electronic versions, photocopies. Anything other than an original wet-ink signature will not be accepted at the border. These letters also must be on official company letterhead, and they also must contain the contact individual for whoever’s writing the letter. It also must be addressed to the border. They also must contain specific details so that the border officer can determine whether or not you qualify for the profession at hand. Start date, end date, what is your profession, what are your duties?
I cannot overemphasize the need for an original. They will not accept anything other than an original wet signature. It also must be coming from a U.S. employer. So if the employer has offices in Canada and in the United States, and the letter is on Canadian letterhead, they won’t accept it. It needs to come from a U.S. based company, and it must have the U.S. contact information, and be addressed from a U.S. based company.
So to answer the question directly, it must be an original ink-signed letter on official company letterhead, and it must be a U.S. based company. Any other letter will be rejected and can cause you problems at the border. Also, the details contained in that letter must be specific. It must be able to appraise that officer of the position, your qualifications, and all the necessary details so they can approve you for a TN visa. If it does not, they will reject you. They could potentially deny you, and also, depending on those details in that letter, it could further complicate your ability to get a TN visa in the future.