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Email Bounce Codes Guide: SMTP Error Codes

Royal Imanli· Published October 27, 2025

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Email Bounce Codes Guide: SMTP Error Codes

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Keep your list clean ✨ and your sender reputation safe 🛡️for your next campaign.

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Keep your list clean ✨ and your sender reputation safe 🛡️for your next campaign.

Frequently Asked Questions

Find answers to common questions about email verification.

What is an SMTP bounce code?

An SMTP bounce code is a numeric message returned by a mail server when an email cannot be delivered. It helps identify the reason for the delivery failure if it’s temporary or permanent.

What does SMTP 550 mean?

The 550 error means the recipient’s server rejected the email. This usually happens when the email address doesn’t exist, your domain is blacklisted, or authentication fails.

What does SMTP 421 mean?

The 421 error is a temporary failure. It often occurs when the recipient’s mail server is busy, overloaded, or temporarily unavailable. The message might be retried later.

What does SMTP 554 mean?

The 554 error typically indicates a permanent failure. It can be caused by spam filters, policy rejections, or the message being flagged as harmful or suspicious.

How do bounce codes affect email deliverability?

Frequent bounces can harm your sender reputation and lower future deliverability. Monitoring and cleaning your email lists regularly helps maintain a strong sender score.

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