SMTP Error 421: Temporary Server Unavailable (How to Fix It)

Open the bounce message or delivery notification. 

Ensure it specifically references variations of: 

421 Service not available 
421 Temporary server unavailable 
421 4.x.x Temporary error 
421 Server busy, try again later 
421 Too many connections 
421 Greylisting enabled 

If the error shows 550, 552, 554, or 535 instead, refer to our SMTP Error Codes guide. 

SMTP 421 is a temporary (soft bounce) error. 

In most cases, it resolves automatically within: 

• 15 to 30 minutes 

Do not repeatedly click “Send.” Instead: 

• Wait at least 15 minutes 
• Retry once 
• Allow automatic server retry attempts to complete 

Repeated sending can increase temporary connection limits. 

If you are sending multiple emails or bulk mail: 

• Reduce simultaneous SMTP connections 
• Send in smaller batches 
• Add delays between sending groups 

High-volume sending is one of the most common triggers for 421 errors. 

Confirm whether the recipient’s mail server is: 

• Under maintenance 
• Experiencing temporary downtime 
• Applying greylisting policies 

Major providers (Gmail, Yahoo, Outlook, AOL) may temporarily defer connections during peak load. 

If the issue affects only one recipient’s domain, the cause is likely on their side. 

Try sending: 

• A plain-text email 
• No attachments 
• No images 
• One recipient only 

If this succeeds, the original message may have triggered temporary rate or filtering sending limits. 

Log in to: 

• cPanel → Email → Track Delivery 
• Plesk → Websites & Domains → Select your domain → Mail → Track Email Delivery 

Check the message status: 

• Deferred = Temporary delay (normal for 421) 
• Failed/Rejected = Different issue (refer to specific SMTP guide) 

421 messages are typically queued and retried automatically. 

Example: 

421 4.2.1 : (DYN:T1) https://postmaster.info.aol.com/errors/421dynt1.html 
421 4.3.1 Out of disk space for temp file 
421 4.3.2 All server ports are busy 

What It Means: 
The recipient server cannot accept messages right now due to temporary resource limitations (disk space, ports, memory). 

How to Fix It: 

• Wait 15 to 30 minutes before retrying the email 
• Reduce the number of emails sent simultaneously if sending in bulk 
• For high-volume senders, stagger or batch messages 
• Check the recipient server status if available 

Reassurance: This is temporary and does not affect your hosting or mailbox. 

Example: 

421 4.4.2 mx.aol.com Error: timeout exceeded 
421 #4.4.5 Too many connections from your host 

What It Means: 
Your mail server attempted to connect, but the recipient server timed out or exceeded concurrent connection limits. 

How to Fix It: 

• Retry sending after a short delay 
• Reduce simultaneous SMTP connections 
• For administrators: review mail queue and delivery logs 

Reassurance: The message is queued or deferred, not lost. 

Example: 

421 4.7.0 mx.aol.com Error: too many errors 
421 4.7.1 POL-T2 Connection refused 
421 Greylisting enabled, please try later 

What It Means: 
The recipient server is using greylisting or temporary policy rules to defer your message, often as an anti-spam measure. 

How to Fix It: 

• Wait for the recommended retry interval (usually 15 to 60 minutes) 
• Ensure your sending server follows proper SPF/DKIM policies 
• Avoid sending large bursts of emails in quick succession 

• Contact the recipient postmaster if persistent delays occur 

Reassurance: These delays are normal for temporary policies, and your server or domain is not blocked. 

Example: 

421 Downstream server error 
421 Fork failed 
421 Unexpected failure, please try later 

What It Means: 
A temporary issue occurred on a downstream server between you and the recipient. 

How to Fix It: 

• Retry sending after waiting 
• Check delivery logs to ensure the email is deferred, not rejected 
• If persistent, contact the recipient’s mail provider for status 

Reassurance: These errors are temporary and do not indicate a problem with your hosting or email configuration.

SMTP 421 errors are often temporary, but repeated occurrences may indicate server-level issues. Review logs to pinpoint patterns: 

grep "421" /var/log/exim_mainlog 
tail -f /var/log/exim_mainlog 

grep "421" /var/log/maillog 
tail -f /var/log/maillog 

Key Insights: 

• Correlate timestamps with high load periods 
• Identify the recipient domains frequently returning 421 
• Determine if errors are temporary resource issues, greylisting, or connection limits 

Deferred messages due to 421 can accumulate. Check the queue to verify delivery status: 

exim -bp 
exim -Mrm <message-id>   # Remove stuck messages if needed

postqueue -p 
postsuper -d <queue-id>  # Delete stuck message if necessary 

Notes: 

• Observe retry intervals and backoff behaviour 
• Avoid mass deletion unless you confirm messages are truly undeliverable 

421 errors often occur due to connection throttling: 

Check current SMTP connections: 

netstat -anp | grep :25 
lsof -i :25 

Adjust server limits (for high-volume senders): 

Exim: /etc/exim/exim.conf – increase smtp_accept_max or smtp_accept_max_per_host 

Postfix: /etc/postfix/main.cf – adjust default_process_limit and smtpd_client_connection_count_limit 

Caution: Changes affect global mail throughout, so always test incrementally. 

Some 421 responses come from recipient servers’ anti-spam greylisting: 

• Temporarily defer messages and retry after 15 to 60 minutes 
• Check SPF/DKIM alignment and ensure your IP has a consistent sending identity 
• Use tools like swaks or telnet to simulate SMTP sending and verify responses: 

swaks --to [email protected] --server mx.example.com --data "Subject: Test\n\nHello" 

telnet mx.example.com 25 

Goal: Confirm retries are accepted, and greylisting policies are obeyed. 

421 errors can sometimes be linked to routing issues or DNS resolution failures: 

• Verify MX records: 

dig MX example.com 

• Test network reachability: 

ping mx.example.com 
traceroute mx.example.com 

• Confirm reverse DNS (PTR) records are correctly set 

• Configure your MTA (Exim/Postfix) with sensible retry intervals: 
• Short delays (15 to 30 minutes) for transient errors 
• Longer exponential backoff for persistent failures 
• Ensure queue processing is not blocked by firewall rules or CPU/memory constraints 

No, it is a temporary rejection. Messages are typically retried automatically. 

Not usually, as 421 errors are temporary and usually server-side. 

Yes, by waiting, checking logs, and adjusting batch sending. Contact support if persistent.