In the closing paragraph of a business letter is it ok to say: I will mail this form to you on or before June 8, 2005.
My concern is saying on or before. Is one of the prepositions unecessary
Can this possibly be used in legal douments? What do you think?
It depends; it's correct if that is what you mean. If you take one out, you will change the meaning. 'On June 8' means not before and 'before June 8' would have the 7th as the last posting date. If you said 'by June 8' that would do the job with one preposition. I can't see why 'on or before' can't be used in a legal document.