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How to Fix Gmail and AOL Email Bounce Errors (550 5.4.300)
Question details
Users are experiencing delayed and rejected emails when sending to established Gmail and AOL contacts, resulting in non-delivery reports.
- Product
- Email Client / Mail Server
- Device & OS
- not provided
- Scenario
- Attempting to send emails to known Gmail or AOL contacts, but the messages bounce back or are significantly delayed.
- Observed behavior
- The recipient server temporarily defers the messages until they expire, returning 550 5.4.300 (expiration) and 421 4.7.28 (unusual sending rate) error codes.
Ensure you have a copy of the complete Non-Delivery Report (NDR) or bounce message ready, as it contains vital diagnostic information for your IT or email administrator.
Verify Domain Authentication Records (SPF, DKIM, DMARC)
The most common cause of these server-side bounce errors is missing, incomplete, or misconfigured email authentication records for your sending domain.
When Gmail or AOL detects an unusual sending rate or cannot verify the sender's identity, they temporarily defer messages. If this delay exceeds the server's limit, a 550 5.4.300 expiration error is generated.
Locate the full bounce message in your inbox and check for specific references to domain reputation or authentication failures.
Forward the complete bounce message to your email or IT administrator for server-level troubleshooting.
Ask the administrator to ensure your domain's SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC records are correctly published and aligned in your DNS settings.
Monitor Domain Reputation with Google Postmaster Tools
Use Google's dedicated diagnostic tools to identify if your sending IP or domain has been flagged for spam-like behavior or unusual sending rates.
Investigate Account Security and Use Alternative Contact Methods
Rule out unauthorized access that might be sending spam from your domain, and find temporary ways to reach your urgent contacts.
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Frequently Asked Questions
What does error 550 5.4.300 mean in Gmail and AOL?
This error means that the recipient's email server (Gmail or AOL) temporarily delayed your message for too long, causing it to expire and bounce back. It typically indicates domain authentication issues (missing SPF/DKIM) or poor IP reputation.
Can clearing Outlook's autocomplete cache fix the 421 4.7.28 warning?
No, clearing the autocomplete cache will not resolve this issue. The 421 and 550 errors are server-side problems related to domain reputation, sending limits, or missing authentication records, rather than a local client cache issue.
How do I fix the 421 4.7.28 unusual sending rate warning?
You must slow down your email sending volume and ensure your domain has properly configured SPF, DKIM, and DMARC records. Registering your domain with Google Postmaster Tools can help your IT administrator monitor and improve your domain's sending reputation.
