How do HTML email size, TLD, attachments, and domain reputation affect deliverability and open rates?

Summary

Email deliverability and open rates are influenced by a multitude of factors, encompassing technical configurations, content practices, and sender reputation. HTML email size, while not directly impacting open rates due to ESP processing of conditional blocks, can affect deliverability due to longer loading times. Attaching files, especially PDFs, increases email size and can trigger spam filters. The choice of TLD has minimal impact if legitimate, although country-specific TLDs may enhance regional deliverability. Domain age and reputation significantly affect deliverability, with newer domains facing initial challenges. Sender reputation is crucial, impacted by sending volume, bounce rates, spam complaints, and engagement. Email authentication (DKIM, SPF, DMARC) is vital for verifying sender identity, and maintaining a clean IP reputation avoids blacklisting. HTML coding quality and the use of inline CSS also play a role, while optimizing email size for mobile users ensures better engagement. Analysing open rates by MX or recipient domain can help diagnose sending issues, and avoiding unsolicited or spammy attachments is essential for maintaining a positive sender reputation.

Key findings

  • HTML Size Impact: Larger HTML emails increase loading times and may negatively impact deliverability, especially for mobile users.
  • TLD Relevance: Using country-specific TLDs can improve deliverability in those regions.
  • Reputation Matters: A positive sender and IP reputation is vital for email deliverability and avoiding blacklists.
  • Authentication is Key: Implementing DKIM, SPF, and DMARC greatly improves deliverability by verifying sender identity.
  • Attachments & Deliverability: Attachments, particularly PDFs and executables, can trigger spam filters and hurt deliverability.
  • Code Quality Counts: Poorly coded HTML can affect rendering and deliverability.
  • Domain Age Benefit: Older domains with established sending histories often have better reputations.
  • Open Rate Insights: Analysing open rates by MX or recipient domain provides deeper insight into deliverability issues.

Key considerations

  • Reduce Email Size: Minimize HTML and attachment sizes for faster loading and better user experience.
  • Targeted TLD: Consider using country-specific TLDs for regional campaigns to improve trust and deliverability.
  • Protect Reputation: Maintain a clean sender reputation by following best practices and monitoring sending metrics.
  • Authenticate Email: Implement and maintain DKIM, SPF, and DMARC for enhanced deliverability and security.
  • Attachment Awareness: Avoid sending unsolicited attachments and be mindful of file types to minimize spam triggers.
  • Clean Code: Ensure clean, well-structured HTML code to avoid rendering issues and spam flags.
  • Monitor Performance: Actively monitor sending metrics and adjust practices as needed.
  • Gradual Domain Warm-up: When using a new domain, implement a gradual ramp-up strategy to build a positive sending reputation.
  • Mobile friendly: Make sure your email is mobile friendly to engage a wider audience.

What email marketers say
10Marketer opinions

Several factors influence email deliverability and open rates. Larger HTML email sizes can negatively impact deliverability due to longer loading times and potential recipient frustration. Poorly coded HTML and unsolicited attachments can also trigger spam filters, reducing deliverability. A positive sender reputation, built through consistent sending practices, low bounce rates, minimal spam complaints, and strong engagement, is crucial. Older domains with established sending histories often have better reputations. Using reputable TLDs, implementing email authentication methods, and maintaining a clean IP reputation further enhance deliverability. Additionally, tailoring email size for mobile users, using inline CSS, and considering country-specific TLDs can improve engagement and trust with recipients.

Key opinions

  • HTML Size: Larger HTML email sizes negatively impact deliverability due to longer loading times.
  • Sender Reputation: A positive sender reputation is crucial for email deliverability.
  • TLD Impact: Using reputable TLDs (e.g., .com, .net, .org) is generally recommended for better deliverability.
  • HTML coding: Poorly coded HTML can negatively impact deliverability.
  • Domain Age: Older domains tend to have better email deliverability as they had time to build positive engagement.
  • Attachements: Unsolicited attachements can trigger spam filters

Key considerations

  • Email size: Reduce HTML email size to improve loading times and user engagement.
  • Sender reputation: Consistently monitor and improve sender reputation through positive sending practices.
  • Valid TLDs: Use reputable and valid TLDs for email addresses.
  • Authentication: Implement email authentication methods (SPF, DKIM, DMARC) to verify sender identity.
  • Clean HTML: Ensure clean, well-structured HTML code to reduce spam flags.
  • Mobile optimisation: Optimise email size for mobile users to ensure fast loading and engagement.
  • Warming up domains: Warm-up new sending domains and IPs to build reputation and trust
Marketer view

Email marketer from Quora emphasises that smaller email sizes are vital for mobile users. Large emails can take longer to load, leading to decreased engagement, especially for users with limited data or slow connections.

April 2021 - Quora
Marketer view

Email marketer from Neil Patel answers that older domains with established sending histories often have better reputations. Building a positive sending history takes time, which can improve deliverability rates compared to newer domains.

April 2023 - Neil Patel
Marketer view

Email marketer from Email Geeks explains that switching the domain from fr/de to .com can impact deliverability if the .com domain isn't properly warmed up and doesn't have a good reputation. Also people might be using inbox filters for the fr/de domain.

April 2022 - Email Geeks
Marketer view

Email marketer from Email Marketing Forum shares that using a reputable TLD (like .com, .net, or .org) is generally recommended. Less common or suspicious TLDs might raise flags with spam filters.

February 2025 - Email Marketing Forum
Marketer view

Email marketer from Sender responds that a positive sender reputation is crucial for email deliverability. Factors contributing to sender reputation include sending volume, bounce rates, spam complaints, and engagement. Poor reputation can lead to emails being filtered as spam.

February 2024 - Sender
Marketer view

Email marketer from Stack Overflow discusses how using inline CSS is crucial for HTML emails, as many email clients strip out or ignore linked stylesheets. Using proper and supported CSS ensures the email renders correctly, affecting engagement and deliverability positively.

June 2023 - Stack Overflow
Marketer view

Email marketer from Mailjet shares that larger HTML email sizes can negatively impact deliverability. Larger emails take longer to load, which can frustrate recipients and lead to lower engagement, potentially harming sender reputation.

September 2023 - Mailjet
Marketer view

Email marketer from EmailGeeksForum.net shares that using country-specific TLDs (like .de for Germany or .fr for France) can improve deliverability in those regions because they appear more familiar and trustworthy to local recipients.

June 2023 - EmailGeeksForum.net
Marketer view

Email marketer from Litmus shares that poorly coded HTML can affect email deliverability. Clean, well-structured code improves rendering across different email clients and reduces the likelihood of being flagged as spam.

November 2022 - Litmus
Marketer view

Email marketer from Reddit user u/EmailExpert explains that sender reputation is critical. If your domain is new, start with low volume, closely monitor bounce rates and spam complaints, and gradually increase volume. Consistent, positive engagement builds reputation over time.

November 2023 - Reddit

What the experts say
7Expert opinions

Email deliverability and open rates are influenced by several interconnected factors. While HTML email size due to conditional blocks might not directly impact open rates as ESPs process them, attaching files, especially PDFs like shipping labels, can negatively affect deliverability by increasing email size and triggering spam filters. Domain age and reputation play a significant role, with newer domains facing initial challenges. Monitoring open rates by MX or recipient domain provides better diagnostic insights. Sending unsolicited or spammy attachments degrades sender reputation. Finally, the TLD's impact is considered negligible as long as it is legitimate, although local geoTLDs can be used for branding.

Key opinions

  • HTML Size: Conditional HTML blocks don't directly impact open rates because ESPs process them.
  • Attachments: Attachments, especially PDFs, can negatively impact deliverability.
  • Domain Reputation: Newer domains face deliverability challenges due to lack of established reputation.
  • Open Rate Analysis: Analysing open rates by MX or recipient domain provides better diagnostic insights.
  • TLD Impact: Legitimate TLDs have a negligible effect on deliverability.
  • Negative Impact of Attachements: Sending unsolicited or spammy attachments degrades sender reputation.

Key considerations

  • Attachment Use: Limit the use of attachments, especially PDFs, or consider linking instead.
  • Domain Warm-up: Focus on building a positive reputation over time, especially with new domains.
  • Monitor Open Rates: Monitor open rates by MX or recipient domain to identify specific issues.
  • Attachment best practice: Avoid sending unsolicited or spammy attachments to maintain sender reputation.
  • TLD Strategy: Choose a legitimate TLD, considering local geoTLDs for branding if necessary.
  • Open rate diagnosis: Ensure you use open rates with MX to diagnose if there is an issue. There is no point diagnosing the overall open rate without breaking it down by MX.
Expert view

Expert from Email Geeks shares that the TLD (e.g., .com, .de) should have a negligible effect on deliverability, assuming it's a legitimate TLD. While using a local geoTLD might have marketing benefits for branding, a .com is generally acceptable worldwide.

May 2021 - Email Geeks
Expert view

Expert from Email Geeks responds that if open rates have dropped more significantly on Hotmail/Outlook and GMX, the reason could be the mail going to bulk. He then advises that there are many things that could have changed and image load rates on their own are not enough to action anything from.

October 2024 - Email Geeks
Expert view

Expert from Email Geeks recommends looking at open rates broken down by MX or recipient domain to diagnose sending issues. Changes in open rates at a specific ISP can provide more actionable insights than overall open rates.

January 2022 - Email Geeks
Expert view

Expert from Word to the Wise (Laura Atkins) explains that newer domains often face deliverability challenges due to a lack of established reputation. Building a positive reputation over time is essential for improving deliverability rates.

September 2022 - Word to the Wise
Expert view

Expert from Spamresource highlights that sending attachments can negatively affect sender reputation if abused. Sending unsolicited attachments or attachments that trigger spam filters can lead to blacklisting and reduced deliverability.

October 2023 - Spamresource
Expert view

Expert from Email Geeks explains that the size of the HTML email due to conditional blocks shouldn't impact open rates, as the ESP executes the conditional logic and only sends one block. He suggests other factors might be causing the fluctuation in image loads.

October 2023 - Email Geeks
Expert view

Expert from Email Geeks suggests attaching a shipping label as a PDF can affect deliverability. Attached files increase email size, potentially interfering with image loads at Google and triggering spam filters.

August 2024 - Email Geeks

What the documentation says
5Technical articles

Email deliverability is significantly affected by several technical aspects. Exceeding Gmail's attachment size limits can cause rejections or spam filtering. Using invalid or misconfigured TLDs leads to email rejection due to routing issues. Certain attachment types, like executables, trigger spam filters. Implementing DKIM, SPF, and DMARC improves deliverability by verifying sender identity and reducing spoofing. Maintaining a clean IP reputation is critical, as blacklisting by services like Spamhaus severely impacts deliverability.

Key findings

  • Attachment Size: Exceeding Gmail's attachment size limits can cause email rejection or spam marking.
  • Invalid TLDs: Invalid or misconfigured TLDs lead to email rejection.
  • Attachment Types: Certain attachment types (e.g., .exe) trigger spam filters.
  • Email Authentication: DKIM, SPF, and DMARC improve deliverability by verifying sender identity.
  • IP Reputation: Blacklisting by services like Spamhaus severely impacts deliverability.

Key considerations

  • Limit Attachment Size: Adhere to Gmail's attachment size limits to avoid rejections.
  • Valid TLD Configuration: Ensure TLDs are valid and properly configured for correct routing.
  • Restrict Attachment Types: Avoid sending attachment types that commonly trigger spam filters.
  • Implement Authentication: Implement DKIM, SPF, and DMARC for improved deliverability.
  • Monitor IP Reputation: Monitor IP reputation and promptly address any blacklisting issues.
Technical article

Documentation from Spamhaus explains maintaining a clean IP reputation is crucial for deliverability. Blacklisting on services like Spamhaus can severely impact deliverability, so it's important to monitor and address any issues promptly.

October 2023 - Spamhaus
Technical article

Documentation from RFC Editor specifies the technical standards for email, stating that the TLD (Top-Level Domain) should be valid and properly configured to ensure correct routing and deliverability. Invalid or misconfigured TLDs can lead to email rejection.

November 2022 - RFC Editor
Technical article

Documentation from Google Workspace Admin Help explains that Gmail has attachment size limits that can impact deliverability. Exceeding these limits can cause emails to be rejected or marked as spam.

March 2025 - Google Workspace Admin Help
Technical article

Documentation from DKIM.org explains that implementing email authentication methods like DKIM, SPF, and DMARC helps improve deliverability. These methods verify the sender's identity and reduce the likelihood of emails being spoofed or marked as spam.

June 2021 - DKIM.org
Technical article

Documentation from Microsoft Support explains that certain attachment types are more likely to be flagged as spam. Executable files (.exe), scripts, and other potentially harmful file types can trigger spam filters, affecting deliverability.

December 2023 - Microsoft Support