All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If it is true that your husband took a girlfriend and exchanged e-mails with her, he has indeed committed an evil deed. For more details, review Fatwa 81356 about the prohibition of relations between men and non-Mahram women outside the bond of marriage.
In fact, Islam has clarified the degrees of forbidding evil. One of these degrees is forbidding evil through actions when one is able to do so and there is no expected greater harm. Consequently, your action is correct. If your husband asks you about this, it is permissible for you to use indirect and figurative answers in order to avoid his harm. You should also utilize this occasion in advising him using a good and kind manner.
Moreover, we also advise you to adorn yourself for your husband, take care of him and be a good wife in order for him not to be led by Satan to look at other woman.