Email is key for both personal and professional lives. But when an email doesn’t make it to its recipient, it can cripple the business, lead to missed opportunities, and ultimately loss of revenue.
Server congestion, or SMTP error 421 4.4.5, happens when an email server is overloaded with incoming or outgoing email requests. This might cause lagging in processing the messages or even a total failure of the delivery. This can be damaging to email marketing campaigns, transactional emails, and business communications of businesses and IT teams overall.
In this guide, we’ll discuss what triggers SMTP 421 4.4.5, how to resolve it, and what steps you can take to avoid it in the future. What is the reason SMTP Email Error 421 4.4.5?
What causes SMTP Email Error 421 4.4.5?
The SMTP Error 421 4.4.5 message typically indicates that the email server is busy or has other issues like network failures, too many users sending emails to the server, etc. Below we discuss these contributing factors in detail.
Server-side factors
- High email traffic: An email server may get overwhelmed when there are too many outgoing or incoming email requests than it can handle. Bulk email campaigns, marketing blasts, or automated transactional emails might bombard the server. Congestion can occur due to sudden bursts of email volume (for example, announcements or newsletters).
- Not enough CPU, memory, or bandwidth: If the mail server has insufficient resources (e.g., CPU, RAM, storage, or bandwidth), the server cannot process emails correctly. Since shared hosting environments typically have strict resource limits, this can worsen congestion problems. Heavy attachment files or high concurrent email connections may also slow down performance.
- Misconfigured mail queues: Undelivered emails can pile up in the server’s email queue if it is not configured correctly, causing delays and backlogs. A backlog may occur if bounce-handling settings are incorrect or if the mail server is repeatedly retrying failed emails. Emails with large attachments, poor email formatting, or spammy content can cause delays or get stuck in the queue.
Network-related issues
- Poor internet connectivity slowing down email transmission. Slow or intermittent internet connections between the mail server and recipient servers can prevent emails from being processed on time. Latency issues can cause email servers to time out while waiting for responses from recipient domains.
- Firewalls or security settings blocking SMTP traffic. SMTP traffic may be blocked by a firewall or intrusion prevention system (IPS), resulting in failure to send emails. Some security settings could mistakenly identify outgoing email traffic as potentially dangerous and interfere with email forwarding it to the recipients. Cloud-based email security tools may also impose rate limits or block bulk messages.
- ISP throttling due to excessive email volume. Internet Service Providers (ISPs) often impose sending limits to control spam. If your email volume exceeds the threshold, emails may be deferred or rejected. Some ISPs throttle based on sender reputation—meaning this is based on too many bounces or spam complaints—and emails may be deferred or rejected. If your emails are being throttled, you may see delays or failed deliveries.
Sender-related problems
- Sending too many emails in a short period. Email providers set daily or hourly sending limits. If you exceed them, your emails may be temporarily rejected. If your IP is new or lacks an established sending history, sending large volumes of emails too quickly can trigger restrictions. Additionally, cold email campaigns without proper warm-up may be flagged as spam.
🔖 Related Reading: How many emails can I send via Gmail and Outlook? - SPF, DKIM or DMARC misconfiguration. If your domain’s Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), or Domain-based Message Authentication, Reporting, and Conformance (DMARC) records are missing or incorrectly configured, email servers may reject your messages. Authentication failures lead to the emails getting into spam or getting blocked by the receiving servers. DNS misconfigurations can make it difficult for recipient servers to verify the legitimacy of your emails.
🔖 Related reading: SPF, DKIM, and DMARC: Boosting Email Security and Deliverability - Using a blacklisted IP address or domain. If your email-sending IP or domain is listed on a spam blacklist, recipient servers may reject your messages. Blacklisting often happens due to sending spam, having too many complaints, or sharing an IP with spam senders. Checking and delisting your domain from blacklists is essential for restoring email deliverability.
How to fix SMTP Email Error 421 4.4.5
The approach to fixing SMTP Error 421 4.4.5 depends on which email provider (Gmail, Yahoo, Outlook, etc.) you are encountering issues with. Below are the best troubleshooting steps for each provider.
For Gmail users
Reduce sending rate and follow Gmail’s email sending limits
Gmail has strict sending limits:
- Free accounts: 500 emails per day.
- Google Workspace accounts: 2,000 emails per day.
If you exceed these limits, your emails may be temporarily deferred or blocked.
Authenticate emails with SPF, DKIM, and DMARC
- Set up SPF records to specify authorized mail servers for your domain.
- Configure DKIM to sign your emails digitally, ensuring integrity.
- Implement DMARC policies to monitor and protect against spoofing attacks.
For Yahoo! users
Check if your domain is blacklisted and request removal if necessary
- Yahoo uses spam blacklists to filter incoming emails.
- Use tools like MXToolbox or Warmy.io’s blacklist checker to verify if your domain is listed.
- If blacklisted, submit a request for delisting through Yahoo’s Postmaster tools.
Ensure your email server complies with Yahoo’s spam filtering policies
- Yahoo uses aggressive spam filtering, so avoid spammy content (e.g., excessive links, all-caps subject lines).
- Maintain a low bounce rate by regularly cleaning your email lists.
Avoid sending bulk emails from shared IPs
- Yahoo may block emails from low-reputation shared IPs.
- Use a dedicated IP address for better deliverability.
For Outlook users
Monitor Outlook’s sender reputation status and follow best practices
- Microsoft maintains a Smart Network Data Services (SNDS) tool that provides insights into your sender reputation.
- Sign up for SNDS and monitor bounce rates, complaint rates, and spam filtering trends.
Avoid high attachment sizes and spam-like content in emails
- Outlook has a strict attachment size limit (up to 20MB for personal accounts, 150MB for business accounts).
- Avoid HTML-heavy emails or excessive use of promotional keywords, as Outlook filters them as spam.
Contact Microsoft support if your domain is repeatedly blocked
- If your emails consistently bounce with 421 4.4.5 errors, request remediation via Microsoft’s Outlook Postmaster Support.
- Provide logs, headers, and error details when submitting your request.
Preventing SMTP Error 421 4.4.5 in the future
While fixing SMTP Error 421 4.4.5 is essential, preventing it from happening in the first place is even more critical—and definitely doable. By implementing proactive email-sending strategies, optimizing server resources, and ensuring proper authentication, you can minimize the risk of server congestion and avoid unnecessary email delays.
- Use email traffic monitoring tools. Track email volume and server performance using monitoring software. Set up alerts for unusual email spikes to prevent overload.
- Follow ISP sending guidelines. Stay within sending limits to avoid throttling or rejections. Authenticate your domain properly to build a positive sender reputation.
- Optimize email queue management. Regularly clear undelivered emails from the queue. Prioritize time-sensitive emails to prevent delays.
- Upgrade infrastructure as needed. Consider dedicated mail servers for high-volume sending. Use dedicated IP addresses for transactional and marketing emails.
- Leverage an email deliverability tool. Warmy.io helps improve email reputation, prevent deliverability issues, and keep your emails out of the spam folder.
How Warmy.io helps enhance email deliverability and combat server congestion issues
Warmy.io provides a comprehensive set of solutions to enhance email deliverability, optimize sending practices, and minimize the risk of SMTP-related errors, including SMTP Error 421 4.4.5 (Server Congestion).
Reliable email warmup process prevents server overload
One of the main triggers of SMTP 421 4.4.5 is sudden spikes in email sending volume, which can cause email servers to become overwhelmed. Warmy.io’s email warmup feature helps mitigate this by gradually increasing the number of emails sent, mimicking natural email activity. This builds trust with email providers and prevents emails from being flagged as spam or rejected due to excessive sending.
Gradually increasing email volume prevents abrupt sending spikes that can overload mail servers. Plus, the engagement-driven warmup process ensures emails receive positive interactions (opens, clicks, replies) to boost sender reputation.
By warming up new or dormant email addresses before heavy usage, Warmy.io reduces the likelihood of SMTP errors related to sudden email bursts, ensuring consistent and reliable email delivery.
Email deliverability testing and domain health monitoring
Warmy.io’s free email deliverability test helps users identify and fix these issues before they impact campaigns.
- Inbox placement testing: See where your emails land (Inbox, Spam, Promotions tab) across major providers like Gmail, Outlook, and Yahoo! and what percentage lands in which folder.
- Blacklist monitoring: Detect if your sending IP or domain is blacklisted, which could trigger SMTP errors and delivery failures.
SPF, DKIM, DMARC verification: Ensure proper authentication to reduce deferrals and enhance sender credibility.
Advanced seed list for email reputation building
Unlike traditional warmup methods, Warmy’s seed list contains genuine email addresses that simulate real human behavior:
- Emails are opened, read, and clicked, reducing the chances of landing in spam.
- If emails go to spam, they are manually removed and marked as important to improve future deliverability.
By ensuring consistent email engagement, Warmy.io minimizes the risk of SMTP 421 errors, optimizing sender reputation and increasing inbox placement rates.
Free SPF and DMARC record generators for secure authentication
Misconfigured SPF, DKIM, and DMARC records can cause email rejections and SMTP errors. Warmy.io provides free tools to generate and verify these records, ensuring proper email authentication and security.
- SPF Generator: Generates SPF records to authenticate email servers
- DMARC Generator: Protects against spoofing and phishing attacks
By implementing strong authentication policies, you can prevent congestion issues and unnecessary SMTP retries and delivery failures, ensuring emails reach their intended recipients without disruptions.
Ready to fix SMTP errors and improve email deliverability?
Don’t let SMTP errors disrupt your communication. Sign up for a free trial of Warmy.io today (no credit card required) or schedule a free consultation with a deliverability expert to ensure your emails consistently land in the inbox.
