How to Fix OX Outbound Error 450 4.2.0 – Recipient Temporarily Rejected (OXSUS0006_404)

Overview

When sending emails through OX Mail, you may encounter the following error:

450 4.2.0 Recipient temporarily rejected - OXSUS0006_404

This error is generated by the outbound email protection system and usually indicates that the sending account or server has reached one or more outbound sending limitations enforced by the OX mail platform.

Reference: https://postmaster-oxsus.vadesecure.com/outbound_error_codes/#_404

Why This Error Happens

The OXSUS0006_404 error is typically triggered when email sending activity exceeds the allowed rate limits or connection thresholds configured on the OX infrastructure.

The following outbound limitations apply globally and cannot be bypassed:

OX Mail Sending Limits

• 500 emails per hour

• 5,000 emails per day

• Maximum 100 recipients per email

• Maximum 30 concurrent SMTP connections

If any of these limits are exceeded, OX may temporarily reject outgoing emails to protect the platform against spam, abuse, or unusual sending behavior.

Common Causes of Error 450 4.2.0

This issue may occur if:

• A bulk email campaign sends too many messages in a short period

• Mailing lists contain a high number of recipients

• Multiple applications or devices are sending emails simultaneously

• SMTP connections are not closing correctly

• A compromised mailbox is sending spam

• Automated systems are retrying failed messages excessively

How to Reduce or Prevent OXSUS0006_404 Errors

1. Reduce Email Sending Rate

Avoid sending large volumes of emails all at once. Instead:

• Space out email campaigns

• Use scheduled batch sending

• Limit automated notifications per minute/hour

2. Keep Recipient Counts Below Limits

Ensure each email contains fewer than:

• 100 total recipients across TO, CC, and BCC fields

For larger mailing lists, split recipients into smaller batches.

3. Monitor Concurrent SMTP Connections

OX only allows up to:

• 30 simultaneous SMTP connections

If applications open too many connections without closing them properly, temporary rate limiting may occur.

Recommended actions:

• Enable SMTP connection reuse

• Reduce thread counts in mailing software

• Configure queue throttling

4. Check for Compromised Accounts

If the mailbox was compromised, spam activity may trigger outbound protection.

Recommended actions:

• Change mailbox passwords immediately

• Enable two-factor authentication if available

• Scan connected devices for malware

• Review sent mail logs

5. Use Dedicated Email Marketing Platforms

OX Mail is designed primarily for standard business communication, not high-volume bulk marketing.

For newsletters or mass campaigns, consider using specialized transactional or marketing email services such as:

• Mailchimp

• SendGrid

• Amazon SES

• Mailgun

These platforms are optimized for large-scale email delivery and reputation management.

Can These Limits Be Increased?

Unfortunately, these outbound limitations are enforced globally by the OX platform and cannot be manually increased or bypassed.

Current enforced limits include:

Additional Resources

Official OX outbound error documentation:

https://postmaster-oxsus.vadesecure.com/outbound_error_codes/#_404

OX Cloud Service Limitations documentation can also provide additional guidance regarding platform restrictions and sending policies.

Conclusion

The 450 4.2.0 Recipient Temporarily Rejected – OXSUS0006_404 error is generally caused by exceeding OX outbound sending thresholds or triggering anti-abuse protections.

To avoid future issues:

• Send emails gradually

• Limit recipients per message

• Reduce concurrent SMTP sessions

• Monitor mailbox security

• Use dedicated bulk email services for campaigns

Following these best practices will help improve deliverability and reduce outbound rejections.