Standard email metrics like open rate and click rate tell only part of the upsell story. The metrics that matter are upgrade conversion rate (what percentage of recipients upgrade within 30 days), revenue per email sent, and time-to-upgrade after email delivery.
Track the full funnel: email delivered, opened, clicked, pricing page viewed, upgrade initiated, upgrade completed. Drop-offs at each stage tell you different things. Low open rates mean your subject lines need work. High clicks but low conversions suggest the pricing page or offer is the problem, not the email.
Segment your results by customer cohort. Customers who have been with you for 6+ months likely respond differently than 30-day-old customers. Power users respond to different messaging than occasional users. Build these segments into your tracking so you can optimize each email for each audience over time.
A/B test subject lines first since they have the biggest impact on whether your message gets seen at all. Then test the CTA placement and wording. Small changes in how you frame the upgrade offer can produce significant conversion improvements.