Table of Contents
- Introduction
- How SMTP Error Codes Work
- The Most Common SMTP Errors and How to Fix Each One
- 421: Service not available, try again later
- 450: Mailbox unavailable
- 550: Mailbox not found / User unknown
- 551: User not local / Relay denied
- 553: Mailbox name not allowed / Authentication required
- 554: Transaction failed / Message rejected
- Connection timeout (no code returned)
- How to Read SMTP Error Codes in Your Email Platform
- Preventing SMTP Errors Before They Happen
- Other Things You Need to Know About SMTP Errors
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IntroductionHow SMTP Error Codes WorkThe Most Common SMTP Errors and How to Fix Each One421: Service not available, try again later450: Mailbox unavailable550: Mailbox not found / User unknown551: User not local / Relay denied553: Mailbox name not allowed / Authentication required554: Transaction failed / Message rejectedConnection timeout (no code returned)How to Read SMTP Error Codes in Your Email PlatformPreventing SMTP Errors Before They HappenOther Things You Need to Know About SMTP Errors
Introduction
SMTP errors are response codes that email servers return when something goes wrong during the process of sending, relaying, or delivering an email. Every SMTP error contains a three digit code that tells you exactly what failed and whether the failure is temporary or permanent. Understanding these codes is the fastest way to diagnose deliverability problems because they describe the issue at the exact point where it occurred, before your email platform has a chance to obscure it behind a generic "delivery failed" message.
Most senders never look at SMTP error codes. They see "bounced" in their dashboard and move on, or they see "delivered" and assume everything worked. Both reactions miss critical information. A 550 bounce from Gmail means something fundamentally different than a 421 deferral from Outlook, and the fix for each is completely different. Treating all bounces the same way is like treating every car warning light by adding more gas.
If your emails are bouncing, timing out, or silently disappearing, the SMTP error code is the first place to look. Not your subject line. Not your content. The error code.
How SMTP Error Codes Work
SMTP errors follow a standardized numbering system. The first digit tells you the category. The remaining digits narrow down the specific issue.
Codes starting with 2 mean success. A 250 response means the receiving server accepted your message. This is what "delivered" means in your email platform's dashboard.
Codes starting with 4 are temporary failures. The server is telling your sending infrastructure to try again later. Most email platforms retry automatically after a 4xx error, and the message often gets through on a subsequent attempt. But if you're seeing consistent 4xx errors from the same provider, it means that provider is throttling or deferring your email repeatedly, which is an early warning sign of reputation problems.
Codes starting with 5 are permanent failures. The server has rejected your message outright and will not accept it on retry. Every 5xx bounce should trigger immediate investigation because the cause is usually something that affects all your email to that provider, not just the single message that bounced.
The Most Common SMTP Errors and How to Fix Each One
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421: Service not available, try again later
The receiving server is temporarily refusing connections. This happens when the server is overloaded, when you're being rate limited for sending too many messages too quickly, or when the server is undergoing maintenance.
Most sending platforms retry automatically after a 421. If the retries succeed, you don't need to do anything. If you're seeing persistent 421 errors from one provider, reduce your sending volume to that provider temporarily. Gmail and Outlook both throttle senders who exceed their per-hour or per-day connection limits, and the threshold is lower for domains with weak or new reputations.
Running warmups through Mailwarm builds sending reputation, which raises these thresholds over time. A domain with established positive engagement can send at higher volumes before triggering rate limits.
450: Mailbox unavailable
The recipient's mailbox exists but can't accept mail right now. Common causes include a full mailbox, a temporarily suspended account, or a server-side policy that's blocking delivery until a condition is met.
This is a temporary error. Your platform should retry. If retries keep failing for the same recipient over several days, remove the address from your active list and try again in a future campaign. Continuing to send to a consistently unavailable mailbox generates repeated soft bounces that erode your sender reputation.
550: Mailbox not found / User unknown
This is the most common permanent error. The recipient address doesn't exist. The person left the company, the domain expired, or the address was fake from the start.
Every 550 bounce should result in immediate removal from your list. No second chances. No "maybe they'll come back." Continuing to send to addresses that return 550 errors is one of the fastest ways to damage sender reputation because it tells inbox providers you're not maintaining your list.
Mailwarm’s email verification catches invalid addresses before they enter your sending pipeline. Running verification before every campaign eliminates almost 550 bounces.
551: User not local / Relay denied
The receiving server doesn't handle mail for the address you're trying to reach, and it's refusing to forward the message. This typically indicates a DNS misconfiguration where the MX records for the recipient's domain are pointing to the wrong server, or the server has been decommissioned.
Check whether the recipient's domain still has valid MX records. If the domain is active but mail isn't being accepted, the problem is on their end. If the domain is dead, remove the address.
553: Mailbox name not allowed / Authentication required
The sending address format is invalid, or the receiving server requires authentication that your sending infrastructure didn't provide. This often happens when the MAIL FROM address is malformed, when your SPF record doesn't include the server you're sending from, or when the receiving server requires SMTP AUTH and your connection didn't authenticate.
Check your SPF record to confirm it includes every IP and service that sends on behalf of your domain. Verify that your SMTP connection is authenticating properly with valid credentials or OAuth tokens. Invest in tools that perform infrastructure health checks, audit SPF, DKIM, and DMARC configurations, and identify authentication gaps.
554: Transaction failed / Message rejected
The receiving server accepted the connection but rejected the actual message. This is the error you see when spam filters, content policies, or reputation scoring block your email. Gmail returns 554 when your domain reputation is too low, when your authentication fails, or when the message content triggers their abuse filters.
A 554 from a major provider is a serious signal. Check your domain reputation in Google Postmaster Tools. Run your domain and IP through MailX for blacklist status. Review your SPF, DKIM, and DMARC records. If reputation is the cause, reduce volume, clean your list, and run warmup through Mailwarm to rebuild positive engagement signals.
Connection timeout (no code returned)
Your sending server couldn't establish a connection to the receiving server at all. No SMTP conversation happened. This is usually caused by wrong port configuration (trying to connect on port 25 when you should use 587), firewall rules blocking the connection, DNS resolution failures on the recipient's MX records, or the receiving server being down entirely.
Check your port configuration first. Port 587 is the standard for email submission with authentication. Port 25 is for server-to-server relay and most ISPs block it outbound. If port configuration is correct, verify that your firewall isn't blocking outbound SMTP connections, and check whether the recipient's MX records resolve properly.
How to Read SMTP Error Codes in Your Email Platform
Most email platforms don't surface raw SMTP codes directly. They translate them into human-readable bounce categories like "hard bounce" or "soft bounce." But the raw codes are usually available if you know where to look.
In SendGrid, check the Activity feed and click on individual messages to see the SMTP response from the receiving server. In Mailgun, the event logs include the full SMTP response code and message. In Amazon SES, bounce notifications include the diagnostic code. In Google Workspace, the bounce notification email includes the SMTP error in the message body.
If your platform doesn't surface SMTP codes at all, check your mail server logs directly. Every SMTP transaction is logged with the full response from the receiving server.
Preventing SMTP Errors Before They Happen
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- Verify your email lists before every campaign. This eliminates 550 bounces from invalid addresses, which is the single most damaging SMTP error category for sender reputation.
- Keep your authentication records clean. SPF, DKIM, and DMARC misconfigurations cause 553 and 554 errors that affect every email you send, not just individual messages. Mailwarm's infrastructure health check catches these before they reach your audience.
- Warm up new domains before sending at volume. Rate limiting (421 errors) hits hardest on domains with no established reputation. Mailwarm's progressive warmup builds the engagement history that raises your sending thresholds across Gmail, Outlook, and Yahoo.
- Monitor your sending patterns for sudden spikes. Going from 200 emails per day to 5,000 overnight triggers 421 deferrals and can lead to 554 rejections if providers interpret the spike as suspicious.
- Check your blacklist status regularly. A domain or IP listing causes widespread 554 rejections across every provider that references the blacklist. MXToolbox checks all major lists in seconds. Mailwarm includes continuous blacklist monitoring as part of its infrastructure checks.
Other Things You Need to Know About SMTP Errors
Is a 421 error something I need to worry about?
Isolated 421 errors are normal. Servers defer connections for many reasons, and your platform retries automatically. Persistent 421 errors from the same provider indicate that provider is throttling you, which means your sending volume exceeds what your reputation allows. Reduce volume or improve reputation through warmup.
What's the difference between a soft bounce and a hard bounce?
A soft bounce is a temporary failure (4xx codes) where the message may succeed on retry. A hard bounce is a permanent failure (5xx codes) where the message will never be accepted. Hard bounces should trigger immediate address removal. Soft bounces should be monitored, and addresses that consistently soft bounce over multiple campaigns should also be removed.
Can SMTP errors affect my sender reputation?
Yes. High rates of 550 hard bounces tell providers your list is dirty. Repeated 554 rejections indicate persistent authentication or reputation problems. Even soft bounces, if frequent enough, signal to providers that your sending practices need attention.
Why do I get different SMTP errors from different providers?
Each inbox provider runs its own filtering and policy engine. Gmail might return a 554 for the same message that Outlook defers with a 421. The error reflects how that specific provider evaluates your email based on its own reputation data, authentication checks, and content analysis.
My platform shows "delivered" but the recipient says they never got it. What happened?
"Delivered" means the receiving server returned a 250 OK during the SMTP transaction. After acceptance, the server runs additional content filtering, reputation evaluation, and engagement analysis that can route the email to spam, promotions, or a quarantine folder. Your platform has no visibility into post-acceptance filtering. Check Mailwarm's inbox placement monitoring to see where your emails actually land across Gmail, Outlook, and Yahoo.