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When MailChannels Outbound Filtering rejects a message, your mail server returns a non-delivery report (NDR) containing a 550 5.7.1 bounce code with a bracketed tag — for example, [SS], [RBL], or [ESA]. Each tag identifies the specific reason your message was blocked. This page explains what each code means and the steps you can take to resolve it.
The [SS] code means MailChannels detected that your message matches a known spam signature. Common triggers include subject lines or body content containing pharmaceutical terms, prescription medication names, lewd or obscene language, or other keywords strongly associated with unsolicited email. Because this is a 5xx permanent failure, your message was not delivered and will not be retried automatically.If you are the sender:
  1. Review your message content — subject line, body text, images, and any embedded links — for keywords or patterns commonly used in spam.
  2. Check that any URLs in your message point to reputable domains with no history of spam or phishing.
  3. Change your account password to a strong passphrase (at least eight characters, mixing uppercase, lowercase, numbers, and special characters).
  4. Verify that your operating system has the latest security updates applied.
  5. Run a full antivirus and malware scan to confirm that no malware is sending email on your behalf.
If you are a service provider and believe this is a false positive:First confirm the sending account is not compromised before requesting a review. See Report false positives and false negatives for instructions on submitting a report.
The [RBL] code means the IP address or domain used to send your message appears on a reputable global blocklist — an indication that the sending infrastructure has been associated with high-volume spam activity. This can happen because of compromised accounts on a shared server, or because the domain owner has a history of sending abusive mail.To diagnose the issue:Check the IP address and domain listed in the bounce message against public blocklist lookup tools:To resolve the issue:
  1. Identify which blocklist has listed the IP or domain.
  2. Follow that blocklist’s delisting process. For Spamhaus, use the lookup tool linked above to request removal.
  3. Secure the sending account: change passwords, apply OS security updates, and run a full malware scan.
  4. Contact your hosting provider to confirm that other accounts on the same server are not sending spam.
Even after a successful delisting request, propagation to all receivers can take time. If mail continues to bounce after delisting, contact your hosting provider.
The [CS] code means MailChannels’ content filter determined that the body of your message contains spam-like or malicious content. Blocked content typically includes malware, phishing language, known-bad URLs, or patterns associated with unsolicited bulk mail.If you are the sender:File a support request with your hosting provider and include the full bounce message you received.If you are a service provider and believe this is a false positive:You can remediate a [CS] block directly from your Host Console without opening a support ticket:
  1. Log in to the Host Console and navigate to Outbound > Log Search.
  2. Locate the blocked message using the search bar or advanced filters.
  3. Click Not Spam on the message details line to immediately clear the spam classification.
Alternatively, click the Flag icon to open the Report an Issue dialog, describe the false positive, and submit a support ticket. You can also send the Insights bounce link to the sender so they can click Not Spam directly.For full instructions on submitting a false positive report, see Report false positives and false negatives.
The [IRR] code means the sending account attempted to deliver email to an unusually high number of invalid (non-existent) recipient addresses. Sending to a large number of bad addresses is a strong indicator of spam activity and damages the IP reputation of your sending infrastructure. MailChannels temporarily blocks the account to limit the reputational impact while you clean up.What causes this:
  • Stale mailing lists that have not been purged of bounced or lapsed addresses
  • A compromised account being used by a spammer to blast a “dirty” list with many invented addresses
  • Contact lists that were never confirmed through an opt-in process
How to fix it:
  1. Go through your bounce reports (NDRs) and remove every address that returned a permanent delivery failure.
  2. Use an address verification service to validate your list before the next send.
  3. Once the rate of invalid recipients drops, MailChannels will automatically lift the temporary block — no manual intervention is required.
Best practices to avoid recurrence:
  • Use double opt-in (confirmed subscription) when adding contacts to your list. This guarantees each address is valid and the owner consented.
  • Remove invalid addresses from your list immediately after receiving a bounce.
  • Only send to subscribers who explicitly opted in. Contacts who did not opt in are more likely to mark your email as spam, which can trigger additional blocks.
If you did not send to a mailing list recently and still received this error, your account may have been compromised. Contact your hosting provider, change your password immediately, and run a complete malware scan.
The [FBL] code means one or more recipients clicked the Report Spam (or Junk) button on a message you sent. MailChannels participates in feedback loops operated by major email providers — including Microsoft Outlook/Hotmail, Gmail, and AOL Mail — which route these complaints back to the sending infrastructure. When complaint rates exceed an acceptable threshold, MailChannels automatically blocks the sender.Why this can happen to legitimate senders:Recipients sometimes click Report Spam instead of Delete out of convenience, even for messages they originally requested. This means you can receive an FBL block even if you believe you sent nothing wrong.Actions to take:
  • Review your sending practices against the M3AAWG Sender Best Common Practices.
  • Only send messages that recipients would reasonably expect to receive.
  • Do not resume contact with subscribers after many months of silence without a re-engagement campaign.
  • Keep the purpose of each message clear and concise.
  • Place unsubscribe links prominently at both the top and bottom of every message.
How long will the block last?MailChannels uses an adaptive approach. Once complaint rates drop back to acceptable levels, the block lifts automatically. MailChannels does not generally accept requests to expedite the removal of an FBL block.
MailChannels cannot reveal which recipients filed complaints. Disclosing that information would allow spammers to identify and remove complainers from their lists, undermining the feedback loop system.
The [ESA] code is the most serious sender-level block. It is issued after a sending account accumulates multiple temporary blocks and continues to generate problematic traffic. Unlike ratio-based blocks that lift automatically, an ESA block is permanent and must be manually removed before the account can send again.Common causes include sustained spam activity, high complaint rates, large numbers of invalid recipients, or other repeated abuse from the same sending account.If you are the end user:Secure your account before anything else:
  1. Change your password to a strong, unique passphrase (minimum eight characters, mixing upper and lowercase letters, numbers, and special characters).
  2. Apply all pending operating system security updates.
  3. Run a full antivirus and malware scan to eliminate any malware that may have been sending spam without your knowledge.
After securing your account, file a support request with your hosting provider so they can initiate the unblock process on your behalf.If you are an administrator:You can delist a sender blocked with ESA directly from the Host Console without contacting MailChannels Support:
  1. Log in to the Host Console and navigate to Outbound > Log Search.
  2. Locate the blocked message for the affected sender.
  3. Click the Flag icon to open the Report an Issue panel.
  4. Click Delist Sender to immediately remove the block.
Do not delist the sender until you have confirmed the account is secured and is no longer sending spam. Delisting a compromised account without fixing the root cause will result in the account being blocked again. Automatic self-service delisting is limited to once every 30 days per account.
You can also receive ESA block notifications by email. Enable Receive Notifications in your Host Console settings. Each notification includes a direct link to unblock the account, and you can automate the unblock step by POSTing to that URL programmatically.Severity levelsMailChannels assigns a severity level to each block based on the most serious condition currently observed:
LevelDescription
Severity 1SMTP service experiencing severe delays or complete outage; web console completely offline; notifications severely delayed or non-functional; Reseller API completely offline.
Severity 2SMTP service experiencing severe delays; web console sporadically unreachable; notifications delayed; Reseller API intermittently unavailable.
Severity 3Moderate impact to one or more services; some degradation in delivery or console access.
Severity 4Minor or informational issues; no significant impact on email delivery.
The ESA block severity reflects the cumulative abuse level observed from the sending account, and determines the urgency with which the issue should be investigated.
The [FEB] code means your message contained an attachment with a file type that MailChannels blocks to protect recipients from malware, phishing, and other security risks. This policy applies to both inbound and outbound mail and is consistent with the attachment restrictions enforced by Gmail, Microsoft Outlook/Exchange, Yahoo Mail, AOL, and other major providers.Blocked file types include:
CategoryExtensions
Executables and installersexe, msi, appx, appxbundle, apk, dmg, cab
Scripts and macrosjs, mjs, jse, vbs, vbe, vb, ps1, wsf, wsh, wsc, bat, cmd, sct
Shortcuts and control fileslnk, cpl, msc, pif, shb
Virtual disks and imagesiso, img, vhd
Java-related filesjar, jnlp
Diagnostic and config packagesdiagcab, diagcfg, diagpkg
Libraries and system filesdll, sys, lib, xll, vxd
Other high-risk typesade, adp, chm, com, ex, ex_, hta, ins, isp, mde, msp, mst, nsh, scr
Archives (ZIP, RAR, and similar) may also be blocked if they contain any of the above file types.What to do instead:
  • Share files using an approved file-sharing or cloud storage service. Restrict access to the intended recipients, set expiration dates on shared links, and avoid public links unless necessary.
  • Share code samples or scripts as plain text snippets in a developer tool, repository, or collaboration platform rather than as email attachments.
  • Do not attempt to bypass the block by renaming the file extension or using weak archive encryption. These methods are not reliable and may result in additional filtering action.
No exceptions to the attachment block policy are possible. If you believe a file was misclassified, contact support with the original file name, timestamp, sender and recipient addresses, and the full bounce message text.

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Reporting misclassifications

If a legitimate message was blocked, or if spam was delivered, see Report false positives and false negatives for instructions on how to submit a report to MailChannels.