How can i prevent my emails from being read/tapped into?

My girlfriend's ex is tapping into/intercepting/hijacking/hacking my emails and Yahoo IMs and sending them to her. He is a computer guru and works for an engineering company in Alabama. Please share any way i can stop him from doing this. Is there software that i can buy to block his actions? I think he has access to some serious software in order to read my emails and forward them to her. Is there any legal action that i can take against him (i.e. illegal wiretapping, invasion of privacy, defamation of character, all of the above, whatever). Thanxs.