If you have been using MailBaby to manage your outbound email, you are probably already familiar with how we handle filtered messages. Up until now, when a message was blocked, your go-to move was clicking the debug URL in the bounce message to see the massive list of individual spam rules that were triggered.
While that deep-dive data is incredibly useful, digging through a long list of complex spam rules can sometimes feel like searching for a needle in a haystack. You just want to know the bottom line: Why did this bounce, and how do I fix it?
To give you a much clearer picture right off the bat, we are introducing Smart Bounce Codes directly into our SMTP responses. This update cuts through the noise so you can see the primary reason for a bounce at a glance, right before you even click the debug link.
What’s Changing in Your Logs?
Instead of just seeing a generic delivery failure code, MailBaby is now embedding specific three-digit sub-codes (100, 200, 300, and 400) into the bounce messages. We are starting with these four core codes, with more specific categories rolling out in the future.
Here is an example of what a bounce log looks like now:
550 200 This message was classified as rSPAM and may not be delivered https://mail.outboundspamprotection.com/mailinfo? MAIL ID AND USERNAME
Notice that 200 right after the standard 550 error? That is your shortcut. Instead of guessing which rule in the debug URL matters most, that sub-code instantly categorizes the root cause.
The New MailBaby Sub-Code Blueprint
When you spot these codes in your logs, here is exactly what they tell you about the main issue:
Code 100: Compromised Account or Local Blacklist
The Main Issue: Security lockdown. This code triggers when specific rules (like COMPROMISED_EMAIL_FROM_BLACKLIST or LOCAL_BL_FROM) detect that an account is sending unauthorized traffic or has been manually blacklisted.
What it means for you: The sending mailbox or system has likely been compromised by a spammer. You need to secure the account immediately and update the password.
Code 200: SPF Authentication Failure
The Main Issue: Domain misalignment. This code groups together rules like SENDER_FAILS_SPF_SPOOF and REDIS_SPFFAIL_CHECK.
What it means for you: The domain’s DNS settings do not authorize MailBaby to send mail on its behalf. The fix is strictly external: update the domain’s SPF record to include MailBaby so it passes spoofing checks.
Code 300: Malware and Virus Hits
The Main Issue: Dangerous attachments. This code is triggered by backend scanners hitting rules like CLAM_VIRUS or DBL_MALWARE.
What it means for you: The email contains a file attachment flagged as malicious. The content cannot be sent, and the originating device should be scanned for malware.
Code 400: Content Filtering & Fuzzy Matches
The Main Issue: Text and layout triggers. This code groups rules like MAILCOW_FUZZY_DENIED and FUZZY_PLUS_DENIED.
What it means for you: The actual body text, structure, or links inside the email too closely resemble known, repeating spam campaigns. Tweaking the wording, removing suspicious links, or changing the formatting will usually get this through.
How This Improves Your Workflow
Don’t worry—the debug URLs aren’t going anywhere. You can still click the link in the bounce message to review the exact rule weights and technical breakdowns as you always have.
However, by pairing the debug URL with these new 100–400 sub-codes, you no longer have to spend time translating individual rule strings just to get a general diagnosis. The code gives you the category instantly, and the URL gives you the raw data to back it up.
If you ever run into a bounce code that has you stuck, or if you need assistance configuring your records to clear up a Code 200 error, our support team is always here to help you review your logs.
