Can I use emails in court to show that my ex was working for someone else?

There is a lot of information that I don't know that can effect your ability to use this in court. How did you obtain the emails? Did you obtain them lawfully or unlawfully? Did you obtain them from discovery? Are the emails communications between spouses during a marriage? As a general rule, judges don't encourage illegal acts. If you obtained these emails unlawfully, then a judge probably won' let you use them. If the emails are spousal communications, they may not be admissible. If they were obtained properly and not spousal communications or some other privileged relationship, then you may be able to offer them into evidence. However, you must still authenticate the documents. This means that you must establish that they were sent to or from your ex. If you authenticate them as coming from your ex, then they should be admissible. If they were sent to your ex, they may be considered hearsay and not be admissible.