Why It Matters
Bounces directly affect your sender reputation. ISPs track your bounce rate, and if it stays above 2%, they'll start throttling or blocking your emails. Hard bounces are especially damaging because they signal you're sending to addresses that don't exist — a hallmark of purchased or poorly maintained lists.
How It Works
Hard bounces are permanent delivery failures. The address doesn't exist, the domain is invalid, or the server has permanently rejected your mail. These addresses should be removed from your list immediately — never retry them.
Soft bounces are temporary. The mailbox might be full, the server might be down, or you've been temporarily rate-limited. Most email platforms will retry soft bounces a few times over 24-72 hours. If a soft bounce persists across multiple sends, treat it as a hard bounce and remove it.
Bounce codes follow SMTP conventions: 5xx codes are hard bounces, 4xx codes are soft bounces. The specific code tells you what went wrong — 550 is "user not found," 552 is "mailbox full," 421 is "try again later."
Quick Tips
- Remove hard bounces immediately. No exceptions, no second chances.
- Validate email addresses at the point of collection — a simple syntax check catches typos before they become bounces.
- If your bounce rate suddenly spikes, check whether a major provider changed their policies or if a large segment of your list has gone stale.