If you’ve received SMTP Error 450: Mailbox Temporarily Unavailable, it means the receiving mail server temporarily rejected your email.
This guide will:
- Explain what SMTP 450 actually means
- Help you identify the specific cause (temporary mailbox, greylisting, or server load)
- Show you how to fix or monitor the issue safely
- Clarify whether the problem is on your side or the recipient’s
By the end of this guide, you’ll confidently diagnose and resolve SMTP 450 errors without unnecessary support escalation.
What SMTP Error 450 Means
SMTP 450 is a temporary rejection (soft bounce).
Unlike permanent errors (5xx), a 450 response means:
- The receiving server temporarily refused the message
- Your server will usually retry delivery automatically
Common reasons include:
- Greylisting (recipient server filtering unknown senders)
- Mailbox storage limits on the recipient side
- Recipient account temporarily locked or under maintenance
- Server-side resource limitations or high load
Because 450 is temporary, understanding the cause helps prevent unnecessary retries and avoids triggering more blocks.
Why This Matters
Ignoring 450 errors may lead to:
- Delays in message delivery
- Confusion over “failed” emails
- Unnecessary ticket submissions to support
- High-volume sending delays
Correctly interpreting the bounce ensures safe, successful delivery without impacting your domain, IP reputation, or hosting environment.
Step-by-Step Fix for SMTP 450 Error – Temporary Reject / Soft Bounce
Follow these steps in order.
1: Confirm the Full Error Message
Open the bounce notification and confirm it includes variations such as:
450 Mailbox temporarily unavailable
450 4.1.1 Recipient address rejected
450 4.2.0 Greylisted
450 4.3.2 Service currently unavailable
450 4.7.1 Try again laterIf a different SMTP code appears, refer to the correct guide.
2: Wait Before Retrying
Because 450 is temporary:
- Retry sending after 30 to 60 minutes
- Avoid repeated resends as this can trigger greylisting or throttling
- High-volume sending may require batch scheduling
3: Verify Recipient Email Address
- Check for typos or formatting errors
- Confirm the recipient address is active
- Ask the recipient to provide an alternative address if necessary
4: Remove Large Attachments
Large emails can trigger temporary deferrals:
- Compress files (ZIP, 7z)
- Use cloud storage links for large content
- Keep email size under 20 to 25 MB
5: Review Delivery Logs (Basic Level)
Log in to:
- cPanel → Email → Track Delivery
- Plesk → Websites & Domains → Select your domain → Mail → Track Email Delivery
Confirm:
- Messages are deferred, not permanently rejected
- Observe retry attempts and timestamps
6: Ask Recipient to Check Their Mailbox
Temporary rejection may be recipient-side:
- Mailbox full
- Account locked or under maintenance
- Server anti-spam policies
7: Monitor Delivery for 24 Hours
If the email is still not delivered after 24 hours, contact the recipient’s provider
Quick Fixes for Common SMTP 554 Variations
SMTP 450 is a temporary rejection (“soft bounce”). This means the receiving server temporarily refused the message, and it may be retried automatically.
Unlike permanent errors (5xx), 450 usually resolves after waiting or minor adjustments.
1. 450 4.7.1 Greylisted / Try Again Later
Example:
450 4.7.1 Greylisted for 5 minutes
450 4.7.1 Try again later
450 Mail temporarily deferredWhat It Means:
The recipient server is using greylisting or a temporary delay mechanism to filter potential spam. Your message is not rejected permanently; it is being held temporarily.
How to Fix It:
- Wait 15 to 30 minutes and allow automatic retries
- Reduce sending frequency or batch emails if sending in bulk
- Ensure your sending server follows proper SPF/DKIM authentication
It only delays delivery temporarily.
2. 450 4.1.1 Recipient Address Rejected
Example:
450 4.1.1 Recipient address rejected
450 4.1.1 [email protected]: Unverified addressWhat It Means:
The recipient’s email address may be incorrect, inactive, or under temporary verification by the server.
How to Fix It:
- Verify spelling and formatting of the recipient email
- Confirm the address with the recipient
- Request an alternative address if necessary
No permanent impact occurs on your mailbox or hosting as the message is only deferred.
3. 450 Mailbox Unavailable
450 Mailbox temporarily unavailable
450 Sorry, mailbox is locked
450 Mailbox is busy, try again laterWhat It Means:
The recipient’s mailbox is temporarily locked, full, or undergoing maintenance.
How to Fix It:
- Ask the recipient to check their mailbox storage or unlock the account
- Retry sending after the mailbox is available
This error affects email delivery only, not your hosting or domain configuration.
4. 450 4.3.2 Service Currently Unavailable
Example:
450 4.3.2 Service currently unavailable
450 4.3.2 Try again laterWhat It Means:
The recipient server is temporarily overloaded or experiencing issues.
How to Fix It:
- Wait and retry later
- Reduce simultaneous connections if sending multiple emails
This is a temporary condition on the recipient server.
5. 450 Account Limit Exceeded
Example:
450 Account limit exceeded
450 Mailbox quota reachedWhat It Means:
The recipient mailbox has exceeded its storage quota.
How to Fix It:
- Recipient must free space in their mailbox
- Retry sending once sufficient storage is available
Advanced (Optional) Fixes (For Technical Users)
Administrators or Technical Users can diagnose server-level issues, retries, or permanent deferrals.
1. Diagnose Mail Queue
Exim
exim -bp
exim -M <message-id>Postfix
postqueue -p
postcat -q <queue-id>Check for stuck messages or repeated deferrals.
2. Check IP Reputation
- Confirm the sending IP is not on blocklists
- Review SPF, DKIM, DMARC alignment
3. Review SMTP Rate Limits
- High-volume sending can trigger extended deferrals
- Reduce concurrent SMTP connections or batch emails
4. Check Reverse DNS (PTR Record)
- Ensure PTR exists and matches HELO hostname
- Misalignment can trigger greylisting
5. Monitor Mail Logs for Patterns
Exim: /var/log/exim_mainlog
Postfix: /var/log/maillog- Look for repeated 450 entries per recipient
- Identify temporary vs permanent failures
6. Restart Mail Service (Optional)
service exim restart
# or
service postfix restartOnly restart if mail service appears stalled.
Before You Move On
Confirm:
- Confirm the recipient email address is correct and active
- Wait 15 to 30 minutes to allow automatic retries (greylisting or temporary deferral)
- Reduce simultaneous outgoing connections if sending multiple emails
- Ensure email size is within recipient limits
- If using attachments, consider splitting large files or using cloud links
Then resend the message.
If This Didn’t Work
- Retry sending a simple, plain-text email
- Remove or reduce attachments
- Ask the recipient to check mailbox availability and storage
- Verify that SPF, DKIM, and DMARC are correctly configured on your sending domain
- Confirm you’re not exceeding your sending limits or batch sizes
If the issue persists, contact support with:
- Full bounce message
- Time the email was sent
- Sender and recipient addresses
- Any steps already attempted
Common Mistakes to Avoid
- Repeatedly resending the same email immediately
- Assuming blacklisting is the cause
- Modifying DNS unnecessarily
- Ignoring queue or log monitoring
Note: SMTP 450 only affects email delivery, not hosting, domain registration, or stored mailboxes.
Empowering Insight
Greylisting protects recipient servers from spam. Legitimate emails are automatically retried, so patience solves most SMTP 450 errors.
Next Recommended Steps
- Check full bounce details
- Verify recipient address
- Confirm SPF/DKIM records
- Monitor deferred messages
- Review email content and size
FAQs
Q. Is SMTP 450 permanent?
No, it’s a temporary error (soft bounce) that resolves automatically or after simple fixes.
Q. Will this affect my website or hosting?
No, it only impacts the delivery of the email.
Q. Can I undo anything?
No configuration changes are required in most cases.
Q. Is this my fault?
Usually not, as this is typically recipient-side or greylisting rules that often trigger 450.
Additional Resources
SMTP Error Codes
SPF/DKIM Setup Guide
Email Sending Limits Guide
Email Bounce Back Guide
Need Additional Support?
We’re Here to Help:
Understanding SMTP Error 450 and how to fix this doesn’t have to feel technical with this easy-to-reference guide. Stuck? Check out our Scope of Support, and then contact our Support Team for further clarity and guidance (https://1grid.co.za/contact-us/). We’re ready to see how we can help!