Metric
Bounce Rate
The percentage of emails returned as undeliverable — above 2% damages your domain reputation.
Bounce rate is the percentage of outbound emails that can't be delivered. Hard bounces (invalid address) are more damaging than soft bounces (mailbox full).
According to Landbase, bounce rates above 2% drop your entire domain's inbox placement by 15-25%. Good verification (NeverBounce, ZeroBounce, Findymail) before sending is therefore essential.