Definition
Every undelivered email generates a bounce code that tells you why it failed. These codes follow the SMTP standard and help you classify bounces for proper handling. Hard bounces (5xx codes) indicate permanent failures and should be removed from your list. Soft bounces (4xx codes) are temporary and should be retried.
Common Codes
- 550: Mailbox not found or unavailable — hard bounce, remove immediately
- 551: User not local or forwarding failure — hard bounce
- 552: Mailbox full — soft bounce, retry later
- 553: Mailbox name invalid or relay denied — hard bounce
- 450: Temporary failure — soft bounce, retry
- 451: Local processing error — soft bounce, retry
- 452: Insufficient storage — soft bounce, retry
Proper bounce handling requires parsing these codes and routing each type to the correct action: remove, retry, or flag for review.
Related Glossary Terms
AOL Mail for Email Marketers
AOL Mail is a legacy email provider with specific deliverability requirements and rendering quirks, now operating as part of the Yahoo+AOL network under shared infrastructure.
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Bulk Email Marketing
Bulk email marketing is the practice of sending a single email campaign to a large list of subscribers simultaneously, typically for promotional, newsletter, or announcement purposes.