When the support is returned to (from your question, it appears that you are mailing it), keep the returned envelopes or other proof of return; even if you void the checks out, put the money aside, in a separate bank account so you have it available at need. Keep evidence (e.g. emails) of all your attempts to contact her and get a new address, etc. You want to be able to prove when she almost inevitably does seek the support (e.g. goes to court and tries to claim you are not paying it) that you have the money and made good faith efforts to pay it, but that she did not cooperate; that way, your husband should be able to avoid liability.