Unsubscribe Rate is important in email marketing because it shows how many people no longer want to receive your emails. A high rate can indicate poor content, irrelevant targeting, or too frequent messaging which indicates that you need to adjust your email strategy to retain and engage your audience.
The formula for calculating email Unsubscribe Rate is:

For example, 8000 emails were successfully delivered to recipients and 20 recipients unsubscribed from the mailing list after receiving emails. So the Unsubscribe rate calculation will be:

As we can see here, Unsubscribe Rate is 0.25% only, which is within the typical industry range (0.2%–0.5%). If this number increases over time, it may be a sign to review your content, frequency, or audience targeting.