What are the deliverability concerns of found mails that did not contain URLs in HTML tags within quoted-printable MIME-parts?
Michael Ko
Co-founder & CEO, Suped
Published 7 Jun 2025
Updated 18 Aug 2025
6 min read
When we discuss email deliverability, the focus often gravitates toward authentication protocols like SPF, DKIM, and DMARC, or content factors such as spam trigger words and image-to-text ratios. However, a more subtle, yet equally critical, concern sometimes surfaces: found mails that did not contain URLs in HTML tags within quoted-printable MIME-parts.
This rather technical phrase points to a specific formatting issue that can hinder how emails are perceived by mail servers and rendered by email clients. While it might sound niche, ignoring it can lead to frustrating inbox placement problems and a subpar recipient experience. Let's delve into what this means and why it matters for your email program.
How quoted-printable encoding works
Before we dive into the deliverability aspect, it’s important to understand quoted-printable encoding and HTML links. Email content is often sent using MIME (Multipurpose Internet Mail Extensions), which allows for various content types and encodings. Quoted-printable is a common MIME content transfer encoding method used when the email body contains mostly printable ASCII characters, but also needs to include some non-ASCII characters or special characters like '='.
The core of the issue arises when a URL (e.g., https://www.example.com) appears in the HTML part of an email but isn't wrapped in an HTML anchor tag <a href="...">. Email clients typically auto-detect and make raw URLs clickable, but this behavior is not standardized and can vary widely. Within a quoted-printable part, every character outside the standard ASCII range, or certain special characters, is encoded using an equals sign followed by its hexadecimal value. This adds complexity to how links might be interpreted.
For example, a raw URL might look like this in the HTML source versus a properly tagged one:
While an untagged URL might still appear clickable in many modern email clients, relying on this auto-detection feature introduces a layer of inconsistency and risk. Spam filters and mail servers evaluate email content for various indicators of spam or malicious intent. An email that contains URLs not properly enclosed in HTML tags, especially within MIME parts, can be flagged as suspicious.
Inconsistent rendering
Email clients may display untagged URLs differently. Some may render them as plain text, while others make them clickable. This inconsistency can lead to a broken or malformed HTML appearance in some inboxes, directly impacting deliverability due to poor content quality signals.
Mailbox providers (MBPs) employ sophisticated spam filtering techniques, including content analysis and reputation scoring. Emails with improper HTML structure or malformed HTML, such as missing <a> tags for URLs, can trigger spam filters more easily. This is because such formatting issues can be indicative of phishing attempts, spam, or simply poorly constructed emails that don't adhere to best practices.
The combination of unencoded URLs and quoted-printable encoding adds another layer of scrutiny. If the encoding process inadvertently breaks the URL string or makes it harder for filters to parse, it could be perceived as an attempt to obscure content, increasing the likelihood of being marked as spam or blocked. This can impact your sender reputation and lead to blocklisting (or blacklisting).
User experience and analytics concerns
Beyond the technical filtering mechanisms, the way your email content appears to the end-user significantly impacts engagement. An untagged URL, even if clickable, presents as a long string of characters rather than a neatly formatted, descriptive link. This impacts the email's aesthetic appeal and can lead to a less professional perception of your brand.
Raw URL appearance
Visual clutter, long strings of characters, and a less polished look in email clients like Gmail. Reduced readability, especially on mobile devices. Difficulty in quickly identifying what the link refers to.
No reliable click tracking
Without proper HTML anchor tags, your email service provider (ESP) cannot reliably track clicks on those URLs. This means you lose valuable data on recipient engagement, making it harder to optimize your campaigns and understand user behavior.
Low engagement metrics, such as click-through rates, can indirectly signal to mailbox providers that your emails are not valuable or engaging. This can negatively affect your sender reputation, potentially leading to future deliverability issues. We always recommend ensuring all links are properly formatted to maximize both deliverability and engagement.
Ensuring your emails are properly structured is a fundamental aspect of good email marketing. While this specific concern might seem minor, it's these details that contribute to the overall quality and trustworthiness of your email campaigns.
Best practices for URL handling
To avoid the deliverability and user experience pitfalls associated with untagged URLs in quoted-printable MIME parts, consider these best practices:
Always use HTML anchor tags: Ensure all URLs in your HTML emails are wrapped in proper <a href="..."> tags. This guarantees consistent rendering and clickability across various email clients and devices.
Validate your HTML: Regularly check your email templates for coding errors or inconsistencies. This helps prevent unexpected rendering issues or spam filter flags.
Maintain a plain text version: Always include a plain text alternative for your HTML emails. This ensures that even if the HTML fails to render, recipients can still view your content and access all URLs clearly.
Test across clients: Use an email testing tool to preview your emails across various email clients and devices. This helps identify and fix rendering issues before they impact your audience.
While modern ESPs often handle much of the encoding and HTML generation automatically, it's still crucial to be aware of how your content is structured. Regularly reviewing your email's raw source (especially the quoted-printable parts) can help catch any anomalies.
Importance of proper HTML structure
Proper HTML structure is a foundational element of email deliverability. It ensures your emails are consistently rendered, trackable, and less likely to trigger spam filters. Invest time in crafting clean, valid HTML, and always use <a> tags for all URLs to maintain a professional appearance and maximize your email campaign's effectiveness.
Ensuring proper email rendering
Ensuring your emails are properly structured, especially when it comes to URLs within quoted-printable MIME parts, is not merely a technicality. It's a critical component of successful email deliverability and overall campaign performance. By adhering to best practices like consistently using HTML anchor tags and thoroughly testing your email rendering, you can significantly improve inbox placement and user engagement.
Paying attention to these details helps build trust with mailbox providers and recipients alike, ensuring your messages are not just delivered, but also effectively consumed. A robust email strategy means looking at every aspect, from content to underlying technical configurations, to achieve optimal results.
Views from the trenches
Best practices
Always wrap URLs in HTML <a> tags for consistent rendering and reliable click tracking.
Regularly validate email HTML to catch structural errors that could impact deliverability.
Include a robust plain text version of your email to ensure content visibility across all clients.
Common pitfalls
Relying on auto-detection of raw URLs by email clients, which is inconsistent and unreliable.
Failing to track clicks on untagged URLs, leading to incomplete engagement data.
Triggering spam filters due to poorly structured HTML and non-standard URL representations.
Expert tips
Monitor your DMARC reports for any anomalies that might indicate content parsing issues, particularly for emails using quoted-printable encoding.
Educate your content creators and email marketers on the importance of proper HTML link formatting, even within visual editors.
When troubleshooting, check the raw email source to see how URLs appear after encoding and before client rendering.
Expert view
Expert from Email Geeks says that typically, including a URL without the <A HREF...> HTML tag should not be a problem, as most Mail User Agents (MUAs) will automatically make it clickable.
2024-04-08 - Email Geeks
Expert view
Expert from Email Geeks says that if you are working with an ESP-provided template, highlighting the URL and using the 'make link' button or icon should resolve any issues.
Visual clutter, long strings of characters, and a less polished look in email clients like Gmail. Reduced readability, especially on mobile devices. Difficulty in quickly identifying what the link refers to.
