Error Messages

Understand common error messages from email servers and learn how to resolve them. These errors appear in your email logs and bounce notifications.

Common Email Server Errors

550 5.1.1

User Unknown / Mailbox Not Found

The recipient email address does not exist on the destination mail server.

Fix: Verify the recipient address is correct. Remove invalid addresses from your mailing list.

550 5.7.1

Message Rejected / Relay Denied

The receiving server refused to accept the message. This can be due to authentication failure, blacklisting, or policy restrictions.

Fix: Check that your SPF, DKIM, and DMARC records are correctly configured. Verify your IP is not blacklisted.

550 5.7.25

PTR Record Not Found

The sending IP address does not have a valid PTR (reverse DNS) record.

Fix: Contact your hosting provider and request a PTR record for your server IP pointing to your mail hostname. See the PTR Record documentation.

550 5.7.26

SPF / DMARC Authentication Failed

The email failed SPF or DMARC validation. The sending server is not authorized to send email for the domain.

Fix: Ensure your SPF record includes Airsend and all other sending services. Check your DMARC policy. See SPF and DMARC docs.

452 4.2.2

Mailbox Full

The recipient's mailbox has reached its storage quota and cannot accept new messages.

Fix: This is a temporary issue. The system will retry delivery. If the bounce persists over several days, the recipient needs to clear their mailbox.

421 4.7.0

Too Many Connections / Rate Limited

The receiving server is temporarily refusing connections due to too many simultaneous connections or excessive sending rate from your IP.

Fix: Reduce sending rate. Spread email sends over time. Airsend handles rate management automatically, but very large lists may trigger this.

450 4.1.8

Sender Rejected — Bad Reputation

The receiving server rejected the email because of poor sender reputation for your IP or domain.

Fix: Check your domain and IP reputation using Google Postmaster Tools or the Airsend dashboard. Warm up slowly and fix any authentication issues.

553 5.1.3

Invalid Address Format

The recipient email address has an invalid format (e.g., missing @ symbol, invalid characters).

Fix: Verify the email address format. Ensure there are no extra spaces or special characters.

554 5.7.9

DKIM Signature Verification Failed

The DKIM signature on the email could not be verified by the receiving server. The message may have been altered in transit, or the DKIM record is incorrect.

Fix: Verify your DKIM DNS record matches the public key shown in your Airsend dashboard. Check for extra spaces or missing characters. See DKIM docs.

550 5.4.1

Recipient Domain Not Found

The domain in the recipient's email address does not exist or does not have valid MX records.

Fix: Verify the recipient's domain is correct and has active MX records.

Bounce Classification

Email bounces are classified by severity and cause. Understanding the classification helps determine the appropriate action.

Category	Code Range	Examples	Action
Hard Bounce	5.1.x	Invalid address, domain not found	Remove address immediately
Soft Bounce	4.2.x	Mailbox full, server unavailable	Retry; remove after 3–5 failures
Policy Bounce	5.7.x	Auth failure, blacklisted, content rejected	Fix authentication or content issues
Technical Bounce	4.4.x, 5.4.x	DNS failure, routing error, no MX record	Check recipient domain; may be temporary

Automatic Bounce Handling

Airsend automatically processes bounces and adds hard-bounced addresses to a suppression list. These addresses will not receive future emails to protect your sender reputation.