Any email sent through the system is stored in the History section of the Gig/Lead, the related contact, and the related company (if applicable). Mago also embeds an invisible tracker to alert you if the email has been opened or not.
When you look at the email in the history panel, read/unread status is indicated by the color of the icon. A red envelope icon indicates that the email has not been read. Sometimes, for privacy reasons, your contact's email server will not allow open tracking and the indicator will stay red even though they have read and responded to your email.
If the envelope icon is green, the email has definitely been opened.