Summary
When an HTML email successfully lands in internal inboxes but fails to reach external recipients, it points to a common challenge in email deliverability. This discrepancy typically arises because internal email systems often have relaxed filtering rules for emails originating from their own domain, essentially whitelisting them. External email providers, however, employ much stricter anti-spam and security measures, scrutinizing every aspect of an incoming message before it reaches the inbox, or even the spam folder. The issue is rarely HTML itself, but rather how the HTML content interacts with these external security gateways, or other factors related to sender reputation and authentication.
Key findings
- Internal vs. external filtering: Internal email systems frequently bypass or relax many security checks for emails sent within the same organization, meaning an email that passes internally might not meet external standards.
- No bounces: An ESP reporting 100% delivery often means the email was accepted by the recipient's server, but not necessarily placed in the inbox. It could have been silently dropped, quarantined, or sent to a spam folder.
- Content issues: While HTML itself is not the problem, issues like broken image URLs (especially those linked to internal servers or having tracking complications), problematic unsubscribe links, or HTML coding errors can trigger external spam filters.
- Quarantine folders: Many corporate recipients use advanced spam filters, such as ProofPoint, which may send suspicious emails to a general quarantine folder that the end-user cannot access directly.
- Recipient provider dependency: Email filtering rules vary significantly between different providers, so an email performing well with one might struggle with another.
Key considerations
- Test externally: Always test your HTML emails by sending them to various external email addresses (e.g., personal Gmail, Outlook, Yahoo) to observe their actual delivery and rendering.
- Check email content integrity: Use an email preview service to identify and fix any HTML or CSS errors, particularly issues with image URLs or plain text parts, as suggested by Litmus in the slack discussion.
- Investigate missing emails: If emails are not in the inbox or spam, ask recipients to check their quarantine folders or contact their IT department. This is a critical step for diagnosing silent drops.
- Review ESP workflow: Ensure you are using your ESP's recommended method for generating or importing HTML emails, rather than copy-pasting raw HTML that might include internal file paths or tracking conflicts.
- Troubleshoot domain-specific issues: If all affected recipients are on the same domain, this suggests a filter or policy specific to that domain.
What email marketers say
Email marketers often find themselves puzzled when their perfectly designed HTML emails reach internal colleagues but vanish when sent to external recipients. The initial suspicion often falls on the HTML itself. However, seasoned marketers quickly learn that the issue is rarely the presence of HTML, but rather subtle nuances in its implementation or the broader sending environment, particularly when dealing with recipient-side spam filters that treat external mail with far greater scrutiny than internal communications.
Key opinions
- HTML is standard: Most email campaigns today are HTML based, so the format itself is typically not the root cause of delivery failure.
- Content errors matter: Issues with coding, like copy-pasting from Word (which introduces messy HTML), or invalid image URLs, can significantly impact deliverability and rendering, especially to platforms like Outlook.
- Hidden destinations: Even if an email doesn't bounce and isn't in the spam folder, it could be caught in a pre-spam quarantine managed by the recipient's IT department.
- Tracking URL complexity: Copying and reusing URLs from old campaigns might break tracking, or cause issues if the URLs were not meant for external use.
Key considerations
- Ask recipients: Politely request recipients to check their spam or junk folders, and specifically ask if they have a separate quarantine. This can help you understand why emails go to spam.
- Test with personal accounts: Before broad sends, conduct small tests to personal external email addresses (like Gmail or Outlook) to catch potential filtering issues early. This is part of a robust deliverability testing checklist.
- Analyze content errors: If your email rendering service flags issues like problems with the plain text part or invalid image URLs, address these immediately. Even minor HTML errors can lead to rendering as plain text or getting blocked.
- Avoid copying flawed HTML: Do not simply copy HTML from previous campaigns, especially if they had known issues or were designed for a different environment.
Marketer view
Email marketer from Email Geeks suggests that HTML by itself should not be the issue, as most emails are HTML. They indicate that issues might arise from copying HTML code from messages previously flagged as spam or from using externally hosted fonts and images.
Marketer view
Marketer from Spiceworks Community notes that if an email client is set to send in plain text, recipients will only receive plain text, regardless of the HTML. They emphasize checking the email source formatting.
What the experts say
Deliverability experts consistently observe that one of the primary reasons HTML emails are accepted internally but rejected externally stems from how different mail systems—especially corporate ones—apply security policies. Internal networks often employ trust-based rules for their own domains, allowing messages to bypass many layers of scrutiny. External mail, conversely, faces rigorous content analysis, sender reputation checks, and authentication validations by recipient Mail Transfer Agents (MTAs) and security gateways, like ProofPoint, which are designed to protect against a wide array of threats including spam and phishing.
Key opinions
- Internal whitelisting: Corporate email setups typically whitelist (trust) their own domain, meaning emails sent internally may not undergo the same stringent checks as those sent to external addresses.
- Recipient-dependent filtering: Filtering is highly dependent on the recipient's email provider and their specific anti-spam configurations. What passes for one might not for another.
- Quarantine vs. spam: Many organizations use a central quarantine system (like ProofPoint) for highly suspicious emails, which is distinct from an individual's spam folder and often requires IT intervention to access.
- Subtle content issues: While minor HTML issues might not completely scramble an email, they can contribute to a higher spam score and lead to messages being blocked or quarantined by stricter external filters.
Key considerations
- External testing is crucial: Always send test campaigns to known external email addresses, especially generic ones like Gmail, to truly gauge deliverability and how content renders. This is essential for understanding your actual deliverability rate.
- Analyze raw email headers: Viewing the 'show original' or raw source of a delivered email can reveal valuable diagnostic information, including authentication results (SPF, DKIM, DMARC) and headers indicating filtering actions. For more technical solutions to improve deliverability, this is a key step.
- Engage recipient's IT: If a specific external domain is consistently problematic, the most effective next step is to have a recipient contact their IT team. They have access to internal logs and quarantine systems to diagnose the exact reason for non-delivery.
- Understand spam filter behavior: Recognize that external spam filters don't just send to junk; they can silently drop messages that are clearly malicious or highly suspicious. Understanding this helps manage expectations about email filtering behavior.
Expert view
Deliverability expert from Email Geeks suggests that it's rare for content-based issues alone to cause mail to completely vanish. They emphasize that the next steps for diagnosis depend heavily on the content itself, the target domains, and any past issues with those recipients.
Expert view
Expert from SpamResource.com notes that email blocklists (or blacklists) are dynamic and constantly updated. They point out that a domain's or IP's presence on a blocklist can significantly impede external deliverability, even if internal systems accept the email.
What the documentation says
Official email documentation and protocol specifications (like RFCs) implicitly differentiate how internal and external emails are handled. Internal mail flow often operates within a perimeter of trust, where sender authentication and content scrutiny can be less rigorous. Conversely, external inbound email is subject to numerous security layers implemented by Internet Service Providers (ISPs) and corporate mail gateways. These layers enforce strict adherence to email standards, authentication protocols (SPF, DKIM, DMARC), and content best practices to combat spam, malware, and phishing threats.
Key findings
- Separate processing: Email systems are architected to apply different security policies to messages based on their origin (internal vs. external).
- Content validation: External email gateways perform extensive content analysis, including checking HTML structure, linked URLs, and the presence of both HTML and plain text parts (multipart/alternative).
- Image and asset hosting: All assets linked within an HTML email, especially images, must be hosted on publicly accessible servers. Local or internal network paths will fail for external recipients.
- Authentication standards: Adherence to email authentication standards such as SPF, DKIM, and DMARC is critical for external deliverability, ensuring your emails are trusted by recipient servers. This is where DMARC, SPF, and DKIM become vital.
- Message tracing: Tools like message tracing data in Office 365 allow administrators to track the path of an email and identify where it was stopped or quarantined.
Key considerations
- HTML validation: Ensure your HTML email code is valid and well-formed according to industry best practices to maximize compatibility across diverse email clients. This aligns with what RFC 5322 suggests.
- Plain text fallback: Always include a well-formatted plain text version of your email. This is not just for accessibility but also for deliverability, as some spam filters flag emails lacking a proper plain text alternative.
- External asset paths: Double-check that all image, font, and other asset URLs are absolute and point to externally accessible web servers.
- Recipient server policies: Be aware that recipient mail systems, particularly corporate ones, may have specific policies that block or quarantine emails based on content, sender reputation, or even the inclusion of external links, for security reasons. Message tracing data can help.
Technical article
Microsoft documentation on Exchange Online Protection (EOP) specifies that inbound email from external sources undergoes robust filtering for spam, malware, and phishing. This includes content analysis that is far more stringent than for internal messages, explaining why an email might pass internally but fail externally.
Technical article
RFC 2046, which defines the MIME format, mandates that emails with multiple parts, such as HTML and plain text, must include both for maximum compatibility. Failure to properly structure a multipart/alternative message can lead to rendering issues or outright rejection by strict mail servers.
Related resources
11 resources
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