If your ex-husband already submitted a return claiming the child, and you e-file your return after his, your e-filed return will automatically be rejected by the IRS computers. That's why you have to print and mail.
If the IRS questions the dependency exemption on your return, they will send you a letter which will tell you what kind of proof they want. It might be a letter from a school or daycare official or a healthcare provider, something like that.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
