Most taxpayers are eligible to file electronically. However, in some cases, you may have to file by mail. Here are a few examples: You, your spouse, or one of your dependents doesn't have a valid Social Security number. Someone already claimed a dependent you are trying to claim. You're married but filing separately and subject to community property rules. Your return requires forms that can only be filed by mail. You have someone with power of attorney and require the refund to be sent to a third party.