$ man bounce-rate-monitoring

Bounce Rate Monitoring

Tracking the percentage of emails that fail to deliver (hard bounces = email doesn't exist, soft bounces = temporary failure like full inbox or server issue). Hard bounce rate above 5% indicates a data quality or infrastructure problem.

High bounce rates are the fastest way to destroy domain reputation. If 10% of your emails bounce, email providers flag your domain as sending to invalid addresses — a hallmark of spam lists. Once flagged, even valid emails from that domain go to spam. Bounce rate monitoring is the early warning system for data quality issues upstream and infrastructure problems in the sending layer.

I check bounce rates daily for the first week of every new campaign, then weekly once metrics stabilize. In Instantly's dashboard, I monitor per-domain and per-mailbox bounce rates. If any domain exceeds 3% hard bounces, I investigate immediately — usually it's a bad batch of emails from enrichment or an MX routing error. If a specific mailbox is bouncing higher than others on the same domain, the mailbox is the problem. I pull it from the campaign and investigate. Bounce rate is the metric I watch most closely. Not opens. Not replies. Bounces.