Who This Is For / When to Use
Use this when you need to:
Identify why emails are bouncing (authentication, invalid recipients, mailbox issues, blocks, etc.).
Filter bounce issues by domain, provider, email type, or sending IP.
Apply the correct fix (DNS/authentication updates, list cleanup, sender reputation work).
Access the Bounce Classification Dashboard
In Kyrios, go to Settings > Email Services.
Open Bounce Classification.
Understand the Bounce Summary Metrics
The dashboard header provides a quick view of deliverability performance for the selected date range and filters:
Permanent Bounce: Total count of permanent (hard) bounces.
Permanent Bounce Rate: Percentage of sends that permanently bounced.
ESP/ISP Block: Count of blocks enforced by email providers (examples: Outlook, Google Workspace).
Delivered: Total delivered emails.
Delivery Rate: Percentage of sends delivered.
Use these metrics to confirm whether you have a deliverability issue (high bounce rate, high blocks, low delivery rate) before drilling into classifications.
Filter Bounce Data
Use filters to isolate specific deliverability issues.
Click Filters in the Bounce Classification dashboard.
Choose one or more filters:
Sending Domain
Email Service Provider
Email Type
Sending IP
Apply filters to update the summary metrics and the bounce classification table.
Read the Bounce Classification Overview Table
The table breaks down bounces so you can pinpoint the root cause:
Email Service Provider: Provider associated with the bounce (example: Gmail, Google Workspace, Apple).
Category: Bounce reason grouping (example: DMARC, Invalid address, Mailbox Full).
Error Code: SMTP error code (example: 550, 554).
Status Code (Enhanced Code): Enhanced SMTP status code when available (example: 5.7.509).
Definition: Meaning of the bounce and what it implies.
Count (%): Volume and percentage of occurrences in the filtered view.
Bounce Categories, Codes, and Definitions
Blocked
Meaning: Your email was blocked because a policy or rule prevented delivery (examples: mailbox policies, size limits, organization rules).
Codes
554 / 5.2.3 — Blocked due to a delivery policy or rule.
DMARC
Meaning: The message failed DMARC authentication because the “From” domain does not align with the SPF/DKIM-verified domains.
Resolution tips
Ensure the From address aligns with the sending domain.
Confirm SPF and DKIM are correctly configured for the sending domain.
Confirm DMARC is configured and matches your desired enforcement policy.
Codes
509 / 5.7.509 — Rejected for failing DMARC alignment/authentication.
550 / 5.0.0 — Rejected; DMARC failed.
550 / 5.7.20 — Rejected; DMARC failed.
550 / 5.7.26 — Rejected; DMARC failed.
550 / 5.7.5 — Rejected; DMARC failed.
550 / 5.7.509 — Rejected; DMARC failed.
550 / 5.7.6 — Rejected; DMARC failed.
550 / 5.7.8 — Rejected; DMARC failed.
550 / 5.7.9 — Rejected; DMARC failed.
550 / Missing — Rejected; DMARC failed; enhanced code not provided.
553 / - — Rejected; DMARC failed; enhanced code not provided.
554 / 5.0.0 — Rejected; DMARC failed.
554 / 5.7.5 — Rejected; DMARC failed.
555 / 5.7.5 — Rejected; DMARC failed.
Invalid address
Meaning: The destination email address is invalid, non-existent, or incorrectly formatted.
Resolution tips
Use email validation before sending.
Remove invalid addresses from your lists.
If the address is “already on the bounce list,” revalidate before retrying.
Codes
450 / 4.1.1
500 / 5.0.0
500 / 5.1.1
501 / 5.1.1
501 / 5.1.3
501 / 5.5.4
501 / 5.7.0
505 / -
508 / 5.1.1
511 / 5.1.1
511 / -
521 / 5.3.2
522 / 5.7.1
525 / 5.7.13
540 / 5.7.1
542 / -
550 / 5.0.0
550 / 5.0.1
550 / 5.0.350
550 / 5.1.0
550 / 5.1.1
550 / 5.1.10
550 / 5.1.181
550 / 5.1.2
550 / 5.1.351
550 / 5.1.6
550 / 5.2.1 (inactive or invalid account)
550 / 5.2.14
550 / 5.2.5
550 / 5.3.0
550 / 5.3.2
550 / 5.4.310
550 / 5.4.312
550 / 5.4.350
550 / 5.4.355
550 / 5.5.0
550 / 5.5.1
550 / 5.5.2
550 / 5.7.0
550 / 5.7.1
550 / 5.7.100
550 / 5.7.13
550 / 5.8.1
550 / - (multiple invalid-address variants, including “already on bounce list” and recipient MX issues)
551 / 5.1.1
551 / -
552 / 5.1.1
553 / 5.1.1
553 / 5.1.3
553 / 5.3.0
553 / 5.5.3
553 / 5.7.1
553 / 5.7.2
554 / 5.0.0
554 / 5.1.1
554 / 5.1.2
554 / 5.2.1
554 / 5.2.2
554 / 5.5.1
554 / 5.7.1
554 / 5.7.7
554 / -
556 / 5.1.1
572 / -
577 / 5.1.1
585 / 5.1.1
605 / - (already on bounce list)
612 / - (recipient-side MX misconfiguration/outage)
Mailbox
Meaning: The domain does not have an active mailbox configured to accept mail.
Resolution tips
Validate the recipient’s email address (including the domain).
Confirm the recipient uses an active mailbox for that address.
Codes
450 / 4.2.2
452 / -
498 / -
511 / -
516 / 5.1.6
522 / 5.2.2
550 / 4.2.2
550 / 5.0.350
550 / 5.2.0
550 / 5.2.2
550 / 5.4.6
550 / 5.5.0
550 / 5.5.1
552 / 5.1.1
552 / 5.2.1
552 / 5.5.2
552 / -
553 / 5.3.1
554 / 5.1.1
Mailbox Full
Meaning: The recipient mailbox is out of storage and cannot accept new messages (often temporary).
Resolution tips
Retry later.
Ask the recipient to clear mailbox space or increase storage.
Codes
452 / 4.1.1
452 / 4.2.2
550 / 5.2.2
552 / 5.2.2
552 / -
Rate Limit
Meaning: The recipient server is temporarily denying delivery because it is receiving too many emails too quickly.
Resolution tips
Slow down sending volume.
Send in smaller batches.
Use alternative contact methods for urgent outreach.
Codes
221 / -
550 / 4.2.1
550 / 5.2.1
550 / 5.2.122
554 / 5.2.121
554 / 5.2.122
Recipient Error
Meaning: The recipient server denied delivery due to recipient-side restrictions, policy controls, connectivity failures, or tenant limits.
Resolution tips
Ask the recipient to allow external email (if they restrict outside-domain mail).
If “expired after retries,” confirm the recipient mail server is reachable and accepting connections.
If this is provider-specific, filter by provider to see scope and patterns.
Codes
450 / -
502 / -
503 / -
510 / -
521 / 5.3.0 (Cloudflare routing/processing issue)
530 / 5.3.0
530 / 5.7.0
530 / 5.7.1
530 / 5.7.57
530 / -
535 / -
541 / 5.4.1
550 / 4.4.0
550 / 4.4.2
550 / 4.4.7
550 / 5.1.3
550 / 5.1.9
550 / 5.3.5
550 / 5.4.103
550 / 5.4.108
550 / 5.4.142
550 / 5.4.300
550 / 5.4.315
550 / 5.4.316 (expired after retries)
550 / 5.4.317 (expired after retries)
550 / 5.4.4
550 / 5.4.7 (permanently rejected after extended delivery attempts)
550 / 5.5.5
550 / 5.7.105
550 / 5.7.124
550 / 5.7.128
550 / 5.7.129
550 / 5.7.133
550 / 5.7.134
550 / 5.7.136
550 / 5.7.193
550 / 5.7.230
550 / 5.7.367
550 / 5.7.51
550 / 5.7.520
550 / 5.7.54
550 / 5.7.64
550 / 5.7.703
550 / 5.7.705 (recipient tenant limit exceeded)
550 / 5.7.900
550 / 5.7.902
550 / 5.7.929
550 / 5.7.987
550 / 5.7.999
550 / 5.8.2
551 / -
552 / 5.7.1
553 / 5.0.0
553 / 5.1.8
554 / 4.3.5
554 / 5.1.0
554 / 5.1.2
554 / 5.2.0
554 / 5.4.6
554 / 5.5.4
554 / 5.7.1
554 / 5.7.105
555 / 5.7.1 (recipient platform not supported / restricted)
556 / 5.7.1
556 / -
571 / -
577 / 5.7.7
579 / -
701 / -
Rejection
Meaning: The email was rejected due to sending-domain DNS problems, message policy limitations, message size, hop limits, or mail loops.
Resolution tips
Verify sending domain DNS (SPF, DKIM, DMARC) is present and valid.
Confirm the sending domain is verified and aligned with the From address.
Avoid mail loops and ensure correct routing.
Reduce message size if too large.
Codes
450 / 4.1.8 (sending domain DNS records invalid)
501 / 5.1.0 (sending domain DNS records invalid)
501 / 5.1.7 (sending domain DNS records invalid)
501 / 5.1.8 (sending domain DNS records invalid)
501 / 5.2.0 (sending domain DNS records invalid)
501 / NA (sending domain DNS records invalid)
533 / 5.7.1 (sending domain DNS records invalid)
550 / 5.1.0 (sending domain DNS records invalid)
550 / 5.1.7 (sending domain DNS records invalid)
550 / 5.1.8 (sending domain DNS records invalid)
550 / 5.2.3 (message too large)
550 / 5.4.0 (too many hops / possible mail loop)
550 / 5.4.12 (too many hops / possible mail loop)
550 / 5.4.14 (too many hops / possible mail loop)
550 / 5.4.6 (mail loop detected)
550 / 5.5.2 (sending domain DNS records invalid)
550 / 5.7.13 (sending domain DNS records invalid)
550 / 5.7.21 (sending domain DNS records invalid)
550 / 5.7.23 (sending domain DNS records invalid)
550 / 5.7.24 (sending domain DNS records invalid)
550 / 5.7.26 (sending domain DNS records invalid)
550 / 5.7.27 (sending domain DNS records invalid)
550 / 5.7.368 (sending domain DNS records invalid)
550 / 5.7.57 (sending domain DNS records invalid)
553 / 5.1.8 (sending domain DNS records invalid)
554 / 5.1.8 (sending domain DNS records invalid)
554 / NA (MAIL FROM domain invalid or poor domain reputation)
Common Issues and Fixes
Fix “Potential Spam / Malicious Activity” or Sender Reputation Issues
Use these steps when you see blocks or deliverability failures tied to reputation signals.
Check IP and domain blacklists
Check whether your sending IP or domain is listed.
If listed, complete the delisting steps required by that blacklist provider.
Verify DNS configuration
Confirm these records exist and are valid for your sending domain:
SPF
DKIM
DMARC
Monitor reputation and deliverability health
Track deliverability indicators for major providers (example: Microsoft and Google tooling).
Ramp sending volume gradually
If you’re using a new or inactive sending domain/IP, increase volume in smaller batches over time.
Maintain a clean email list
Remove inactive/invalid email addresses to reduce bounces.
Remove contacts who generate repeated bounces.
Follow best practices for email content
Avoid spam-like subject lines and formatting.
Balance text and images (avoid image-only emails).
Ensure links are safe and reputable.
Include an unsubscribe link in marketing emails.
FAQ
What is Email Bounce Classification?
Email Bounce Classification is a Kyrios feature that identifies and categorizes bounced emails by cause, so you can analyze delivery issues and improve deliverability.
How can I access the Bounce Classification feature?
Go to Settings > Email Services in Kyrios, then open Bounce Classification.
What are the main categories of email bounces?
Common categories include Mailbox Full, DMARC, Invalid address, Mailbox, Rate Limit, Recipient Error, Rejection, and Blocked.
How can I filter bounces to investigate specific issues?
Use the dashboard filters to narrow results by Sending domain, Email service provider, Email type, or Sending IP.
What insights does the Bounce Summary provide?
The Bounce Summary shows key deliverability metrics: Permanent Bounce, Permanent Bounce Rate, ESP/ISP Block, Delivered, and Delivery Rate.
How do I resolve issues shown in the bounce details?
Use the category and definition to choose the correct fix:
DNS/Auth issues: Validate SPF/DKIM/DMARC and domain alignment.
Invalid address / Mailbox: Validate emails and remove bad addresses.
ISP/ESP blocks: Address policy/reputation issues and reduce risky sending behavior.
Can I track bounces by individual email service providers?
Yes. The dashboard breaks down bounces by provider so you can isolate provider-specific issues.
What is the difference between an “Invalid address” and a “Mailbox” error?
Invalid address means the email address is non-existent or malformed. Mailbox means the domain does not have an active mailbox configured to accept mail at that destination.