mail it without any proof. such proof would just be ignored. in any case, the IRS would process your return as you were the one who could claim the dependent. in about a year the IRS sends both parties a request to amend (to remove the dependent claimed). if neither does, then the IRS sends a second letter to both parties asking for proof they're the ones who can claim the dependent. the letter will contain suggestions on what should be submitted as proof. from the submissions or lack thereof, the IRS sends a bill to the party that loses.
