If the letter has been written as part of somebody's job - for instance, a letter from the Personnel Officer to a candidate saying "the job's yours", then its copyright belongs to the employer. Any letter which is written on behalf of an employer, for the employer's purposes, which would not have been written except for the existence of the employment-relationship, is the copyright of the employer. A personal letter is the copyright of the writer and may not be published without the express permission of the writer, never mind who has received it. Putting the text of the letter on a website without permission from the writer of the text would be breach of copyright, and could result in a considerable fine. If the publication could be considered malicious, there could even be a penalty of imprisonment, if the foreseeable consequences were bad enough.