Do email HTML templates affect deliverability?

Key findings

• Template changes: Shifting from an old HTML template to a new one, even with the same IP address, can cause significant deliverability issues, as observed with major mailbox providers like BTInternet.
• Spam classification: If a new template is causing bounces, the bounce message often indicates the email looks like spam. This highlights how content and code affect filtering.
• File size: Large HTML file sizes (typically over 100KB) can negatively impact deliverability and lead to worse spam test scores. Mailjet notes file size directly impacts deliverability.
• Code quality: Malformed HTML can lead to rendering issues and potentially trigger spam filters, affecting how your messages are handled by recipients' inboxes. Learn more about malformed HTML and deliverability.
• Images and links: While images themselves don't inherently harm deliverability, an email composed solely of images is detrimental. Similarly, the reputation of tracking links and image hosting domains can impact deliverability.

Key considerations

• Template hygiene: Regularly check your HTML templates for excessive spacing, blank table rows, or hidden text that could be flagged by spam filters.
• Testing new templates: Always thoroughly test new email templates, especially when changing email service providers or sending infrastructure. This includes checking for potential spam triggers.
• Domain and IP reputation: Even with a good template, a new or poor IP address or subdomain reputation can cause deliverability issues. Always consider the impact of your email tool on deliverability.
• Bounce analysis: Pay close attention to bounce messages to understand the specific reasons for non-delivery. This can provide clues about template-related problems.

Key opinions

• Template sensitivity: Marketers report that changing an email HTML template can directly impact deliverability, even when other variables like the sending IP remain constant.
• Bounce reasons: Bounce messages often indicate that the new template is being flagged as spam, suggesting content or code quality issues are at play.
• Hidden elements: Elements like increased spacing, blank table rows, or hidden text within templates are believed to negatively impact deliverability.
• Image-only emails: Some marketers still question the use of images. While images generally don't affect deliverability, image-only emails are bad for marketing and deliverability.
• HTML vs. plain text: Mailmunch highlights that email format impacts deliverability, overall campaign success, and conversions. While HTML is widely used, poorly designed HTML can be an issue.

Key considerations

• Thorough testing: Before full deployment, marketers should send test emails to various clients and providers to catch unexpected deliverability issues stemming from new templates.
• Image optimization: Optimize images for web and email to reduce file size. For more information, check do images in emails affect deliverability.
• Link reputation: Ensure any new link tracking domains or image hosting domains used by the new email builder have a good reputation and are not on any blocklist (or blacklist) a recipient might use. See HTTP links and deliverability.
• HTML code review: Perform a thorough review of the HTML code for cleanliness, avoiding unnecessary elements that could inflate file size or look suspicious to filters.

Key opinions

• Indirect impact: While HTML itself isn't a direct block factor, elements within the HTML, such as large file sizes or problematic link tracking, can affect deliverability.
• Blackholing by ISPs: Deliverability expert from Email Geeks (referencing BT Postmaster) highlights that some ISPs, like BTInternet, might accept mail and then blackhole it without providing bounces, making template-related issues harder to detect.
• Reputation of associated domains: Deliverability expert from Email Geeks emphasizes that the reputation of domains used for link tracking and image hosting within the template is critical. If these domains are blacklisted (or blocklisted) or have poor reputation, it can cause delivery failures.
• Code cleanliness: Experts suggest that email HTML coding affects deliverability, particularly concerning excessive or suspicious code elements.
• New vs. old builder: The transition between email builders and their associated HTML output can reveal underlying deliverability challenges that were not present with older, perhaps simpler, templates. This is especially true for new email templates and Gmail.

Key considerations

• Beyond the IP: While IP and domain reputation are crucial, if deliverability issues persist with a new template on a previously good IP, the template itself needs scrutiny.
• Hidden content check: Experts advise checking for elements like increased spacing, blank table rows, or hidden text that may be inadvertently included by new email builders or inherited from older designs.
• ISP-specific nuances: Recognize that ISPs have varying filtering rules. A template that works for one provider may not for another. This is particularly relevant for Microsoft deliverability.
• Proactive monitoring: Implement robust monitoring of bounce rates and sender reputation when deploying new templates or changing ESPs, as immediate feedback can be limited.

Key findings

• File size limits: Mailjet documentation explicitly states that HTML email files should be 100KB or less to maximize deliverability. Exceeding this limit can cause emails to be truncated or flagged. Larger sizes affect deliverability.
• Spam test scores: Email on Acid's best practices mention that emails above 100KB receive worse spam test scores, indicating that file size is a direct factor in how filters evaluate your content. HTML file sizes affect performance.
• HTML vs. plain text: Dyspatch debunks the myth that plain text emails always have better deliverability, asserting that well-crafted HTML emails get delivered when properly coded and tested. This implies HTML quality is key. For more, see plain text email version affects deliverability.
• Code cleanliness: Serversmtp emphasizes that a neat and carefully coded HTML email template ensures a better reading experience and contributes to deliverability. Poorly structured or bloated code can be problematic. Email code quality impacts deliverability.
• Image-only emails: Tower Marketing documentation identifies image-only emails as detrimental, causing deliverability issues and slow loading times, because filters often cannot scan the content of images. This is relevant to are image-only emails bad.

Key considerations

• Minimize file size: Adhere to recommended HTML file size limits, typically around 100KB, to avoid truncation and spam flagging. This includes optimizing images and reducing unnecessary code.
• Clean code practices: Ensure your HTML is well-structured, semantic, and free of extraneous code, excessive comments, or deprecated tags that could be misinterpreted by email clients or filters.
• Content balance: Avoid image-only emails and ensure a healthy text-to-image ratio. This helps filters understand your content and provides a better fallback for recipients with images blocked.
• Cross-client compatibility: Design HTML templates with responsiveness and compatibility across various email clients in mind. Inconsistent rendering can reflect poorly on your sender reputation.
• Avoid suspicious elements: Be cautious with JavaScript, web fonts (particularly to Microsoft or Yahoo), or excessive CSS that could trigger spam filters.