We have hundreds of bounces, can't tell which are permanent
SMTP codes split temporary (4xx) from permanent (5xx). Hard bounces (5xx) are permanent: mailbox doesn't exist, domain doesn't exist, policy-rejected. Soft bounces (4xx) are temporary: mailbox full, server unreachable, throttling. Suppress hard bounces immediately, suppress soft after X consecutive fails.
Try this first
- 1Read the DSN (Delivery Status Notification). The 'Status:' line (RFC 3463) is more readable than the SMTP code. 5.1.1 = mailbox doesn't exist, 5.1.10 = recipient blocked, 4.4.7 = message expired.
- 2Build a suppression list for hard bounces. Remove right after 5xx, not 'maybe next month'.
- 3Soft bounces: accumulate, suppress after for example 5 fails over 7 days. Or follow your ESP's recommendation.
- 4Distinguish 'recipient blocked' (5.1.10, 5.7.1, 5.7.606) from 'mailbox not found' (5.1.1). First is reputation, second is data quality.
- 5Enable automatic bounce handling in your ESP. Postmark and SendGrid do suppression by default; self-hosted MTA needs scripts.
When to bring us in
If bounce rate climbs above 2 percent on a send, you're buying reputation damage. Pause and clean the list.
See also
- Our emails land in spam for some recipientsAlmost always an SPF, DKIM, or DMARC setting that is wrong or missing, or a sender name that mimics a well-known brand.
- Someone reports receiving phishing emails "from us"Read: spoofing. Someone is abusing your sender name, not necessarily your actual mailbox.
- An email bounces (NDR): delivery failedThe NDR text usually states the exact reason. Reading it is step one.
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