Transient Errors (Temporary Issues)
These are temporary problems that usually resolve on their own. Our system will automatically retry sending the email multiple times over a period (usually after 24 hours).
Transient-General
Description: A general temporary failure occurred.
Possible Causes:
Temporary server overload or downtime on the recipient’s side
Greylisting or DNS resolution delays
Action: No immediate action required. Wait for retry attempts to complete.
Transient-MailboxFull
Description: The recipient’s inbox is full.
Possible Causes:
The recipient has exceeded their mailbox storage quota
Action: The system will retry. If urgent, consider reaching out through another channel.
Permanent Errors (Delivery Failed)
These indicate that the message cannot be delivered and won't be retried.
ContentRejected
Description: The content of the email was rejected by the recipient's server.
Possible Causes:
Triggered spam filters
Suspicious links, attachments, or formatting
Blocked keywords (e.g., “Free”, “Buy now”)
Action:
Remove or simplify links and attachments
Avoid common spam phrases
Try sending a plain-text version of the message
550 – Mailbox Unavailable / User Not Found
Description: The recipient’s email address is invalid or does not exist.
Possible Causes:
Typo in the email address
The email account has been deleted or deactivated
Action: Double-check the address and contact the recipient if needed.
553 – Mailbox Name Not Allowed
Description: The recipient address format is not allowed.
Possible Causes:
Invalid email format or restricted characters
Action: Correct the address and resend.
552 – Exceeded Storage Allocation
Description: Message size exceeds the recipient's inbox capacity.
Possible Causes:
Large attachments
Action: Reduce the size of attachments or content and resend.
554 – Transaction Failed
Description: A generic permanent failure.
Possible Causes:
Blacklisted sender IP/domain
SPF, DKIM, or DMARC authentication failed
Action:
Check your domain’s email settings
Use mxtoolbox.com to check blacklists
Authentication-Related Failures
Email services use authentication protocols to prevent spam and spoofing. If your domain isn’t configured correctly, emails may fail to deliver.
SPF / DKIM / DMARC Failures
Description: The email server couldn’t verify that your message came from an authorized sender.
Action:
Ensure your domain’s SPF, DKIM, and DMARC records are set up properly.
Contact your domain administrator or hosting provider if you're unsure.
Troubleshooting Tips
Test with a plain-text version of your email
Avoid spammy words and excessive formatting
Use authenticated email domains with proper SPF/DKIM/DMARC records
Contact support if you’re repeatedly seeing the same errors