Bounce Rate (Email)

The percentage of sent emails that are returned undelivered. Hard bounces indicate permanent delivery failures (e.g., SMTP 550 — mailbox not found, invalid domain). Soft bounces are temporary failures (e.g., SMTP 452 — mailbox full, SMTP 421 — server temporarily unavailable). High bounce rates damage sender reputation and can lead to IP blacklisting. Email services typically recommend keeping bounce rates below 2%.