This depends on the campaign you're running. Generally speaking, email marketing includes several key performance indicators (KPIs), such as:
Delivery rate – the percentage of emails you send that actually reach your recipients without being rejected.
Open rate – the percentage of emails your subscribers actually open.
Click-through rate – the percent of opened emails that are engaged with in a way that sends the recipient to your website (e.g., clicking an enticing CTA button).
You can get more granular in how you measure the success of your email marketing efforts. For example, if you're conducting a campaign to get people to sign up for a demo of your software, you need a closed loop reporting approach that attributes bottom-line results (e.g., demo sign-ups, sales) to your email campaigns.