How do you prove an email is legit from the headers?

My husband is in arbitration. He has an email from the Florida Department of Highway proving that his license was valid at the time he was working. Due to a glitch the suspended license was backdated. We have an email from the Department verifying his license was not revoked on the day it shows. We are going to arbitration and submitted the email. The OIG thinks the email is fake. Can we prove it is true from the headers? We can not submit any further information now that a court date has been set.

Thank to all, this means a lot to us!

Tabitha