SMTP 4xx codes are temporary failures (deferrals) — the server is telling you to try again later. SMTP 5xx codes are permanent failures (rejections) — the message will never be accepted. The most common: 421 = server busy/rate limited (wait and retry), 450 = mailbox unavailable temporarily, 550 = mailbox doesn't exist or rejected permanently, 553 = authentication failure. Fix 4xx by reducing send rate. Fix 5xx by checking the specific code against this guide.
SMTP Response Codes: Every 4xx and 5xx Code Explained (With Fixes)
How SMTP Response Codes Work
When your server sends an email, the receiving server responds with a three-digit code. The first digit tells you the category:
- 2xx = Success (message accepted)
- 3xx = Intermediate (need more info — rare in email)
- 4xx = Temporary failure (try again later)
- 5xx = Permanent failure (don't retry)
Most ESPs handle retries automatically for 4xx codes. 5xx codes require action from you.
4xx Temporary Deferrals
These mean "try again." Your sending server should retry automatically.
| Code | Meaning | Common Cause | Fix |
|---|---|---|---|
| 421 | Service not available | Server busy, rate limiting, greylisting | Reduce send rate, wait, retry |
| 421 4.7.0 | Connection rate limited | Sending too many connections too fast | Reduce concurrent connections |
| 450 | Mailbox unavailable | Temporary server issue | Automatic retry usually resolves |
| 450 4.2.1 | Mailbox disabled/full | Recipient's mailbox is full or disabled | Retry; remove if persistent |
| 451 | Local error in processing | Receiving server error | Retry automatically |
| 451 4.3.0 | Mail system full | Receiving server out of resources | Wait and retry |
| 452 | Insufficient storage | Server out of disk space | Wait and retry |
| 452 4.5.3 | Too many recipients | Per-connection recipient limit reached | Reduce recipients per connection |
Gmail-Specific 4xx Codes
| Code | Meaning | Fix |
|---|---|---|
| 421-4.7.0 | IP has sent unusual traffic | Reduce volume, check for compromised account |
| 421-4.7.28 | IP not in Gmail's allowlist | Warm up gradually, improve reputation |
5xx Permanent Rejections
These mean "don't retry." Take specific action based on the code.
| Code | Meaning | Common Cause | Fix |
|---|---|---|---|
| 550 | Mailbox unavailable | Address doesn't exist | Remove from list immediately |
| 550 5.1.1 | Recipient not found | Invalid email address | Remove from list |
| 550 5.1.2 | Domain not found | Domain doesn't exist or has no MX | Remove from list |
| 550 5.2.1 | Mailbox disabled | Account closed/disabled | Remove from list |
| 550 5.4.1 | Relay access denied | Your server isn't authorized | Check SMTP auth config |
| 550 5.7.0 | Rejected by policy | Content or reputation based | Check reputation, review content |
| 550 5.7.1 | Message rejected | Authentication fail, blacklisted, or spam filtered | Check SPF/DKIM/DMARC, check blacklists |
| 550 5.7.25 | IP not authenticated | Sending IP has no PTR record | Set up reverse DNS |
| 550 5.7.26 | DMARC authentication failed | DMARC policy causing rejection | Fix DMARC alignment |
| 551 | User not local | Recipient doesn't exist here | Remove or update address |
| 552 | Message too large | Email exceeds size limit | Reduce attachment/image size |
| 553 | Mailbox name invalid | Syntax error in address | Fix the email address |
| 554 | Transaction failed | General rejection, often spam | Check reputation and authentication |
Gmail-Specific 5xx Codes
| Code | Meaning | Fix |
|---|---|---|
| 550-5.7.1 | Gmail blocked your message | Check domain reputation in Postmaster Tools |
| 550-5.7.26 | Unauthenticated email not accepted | Fix DMARC alignment (SPF or DKIM must align with From: domain) |
| 550-5.7.27 | SPF record doesn't include sending IP | Add the sending IP/service to your SPF record |
Outlook-Specific 5xx Codes
| Code | Meaning | Fix |
|---|---|---|
| 550 5.7.1 | Sender rejected by Outlook | Check IP on Microsoft SNDS, submit sender info form |
| 550 5.7.606 | IP blocked due to spam | Request delisting via Microsoft's forms |
Practitioner note: The most misunderstood bounce: 550 5.7.1 on Gmail. People assume it's content-based, but 90% of the time it's reputation or authentication. Check Google Postmaster Tools for domain reputation before changing a single word of your email.
Practitioner note: If you're seeing 421 deferrals from one specific provider but not others, you're hitting their rate limit. This is common when migrating ESPs or increasing volume without warmup. Slow down sends to that provider specifically.
Practitioner note: When a client tells me "all our emails are bouncing," the first thing I check is whether it's 4xx or 5xx. 4xx means slow down. 5xx means something is configured wrong. The fix is completely different.
If you're seeing rejection codes you can't resolve, schedule a deliverability audit — I'll trace the exact cause and fix the infrastructure.
Sources
- RFC 5321: Simple Mail Transfer Protocol (SMTP response codes)
- Google: Gmail SMTP errors
- Microsoft: NDR error codes
v1.0 · March 2026
Frequently Asked Questions
What does SMTP 550 mean?
550 is a permanent rejection. The most common variants: 550 5.1.1 = recipient doesn't exist (remove from list), 550 5.7.1 = rejected by policy (authentication failure, blacklisted, or content filtered), 550 5.7.26 = DMARC authentication failed.
What does SMTP 421 mean?
421 is a temporary deferral meaning 'try again later.' It usually means the receiving server is busy, you're sending too fast, or you've hit a rate limit. Reduce sending speed and the message should deliver on retry.
Should I remove contacts that get a 4xx bounce?
No. 4xx codes are temporary. Your email server should automatically retry. Only remove contacts after persistent 4xx failures (e.g., 5+ consecutive attempts over 72 hours) or after a 5xx permanent failure.
What does 'high probability of spam' mean in a bounce?
This typically appears as a 550 5.7.1 rejection and means the receiving server's spam filter flagged your message. Common causes: poor sender reputation, missing authentication, content triggers, or blacklisting. Check authentication first, then reputation.
Why am I getting 452 'too many recipients'?
The receiving server limits how many recipients you can address per connection. This is normal throttling. Your email server should automatically open new connections. If using a script or custom sender, limit to 50-100 recipients per SMTP connection.
Want this handled for you?
Free 30-minute strategy call. Walk away with a plan either way.