Is using the letter O instead of the number 0 in email subject lines a good practice?

Summary

The overwhelming consensus from email marketers, deliverability experts, and official documentation is that using the letter 'O' instead of the number '0' in email subject lines is a detrimental practice. Character substitutions can trigger spam filters, harm sender reputation, decrease user trust, reduce accessibility, and make emails appear unprofessional. While some suggest A/B testing, the potential risks generally outweigh any perceived benefits. Clarity, authenticity, and adherence to email marketing best practices are emphasized as key for successful and ethical email communication.

Key findings

  • Spam Trigger: Character substitutions are easily detected by spam filters, leading to decreased deliverability.
  • Reputation Damage: Such practices negatively impact sender reputation, potentially causing emails to be marked as spam.
  • Reduced Trust: Using 'O' instead of '0' is seen as a deceptive tactic, reducing user trust and credibility.
  • Accessibility Issues: Character substitutions can make emails less accessible and readable for all users.
  • Unprofessional Appearance: Emails with these substitutions are perceived as unprofessional and potentially suspicious.

Key considerations

  • Authenticity and Transparency: Prioritize genuine and transparent communication over stylistic tricks.
  • Policy Compliance: Adhere to the anti-spam policies of major email providers like Google and Microsoft.
  • Sender Reputation Management: Actively monitor and maintain a positive sender reputation to ensure optimal deliverability.
  • A/B Testing (Cautiously): If considering this practice, carefully A/B test the impact on deliverability and engagement, but be wary of potential negative consequences.

What email marketers say
11Marketer opinions

The consensus among email marketers and experts is that using the letter 'O' instead of the number '0' in email subject lines is generally a bad practice. This is due to concerns about accessibility, deliverability, and professionalism. While some suggest A/B testing, most agree the risks outweigh any potential benefits, as such substitutions can trigger spam filters, reduce readability, annoy subscribers, and harm sender reputation. Clear and genuine communication is prioritized over potentially deceptive character replacements.

Key opinions

  • Deliverability Impact: Character substitutions can trigger spam filters and negatively affect sender reputation.
  • Accessibility Concerns: Using 'O' instead of '0' can reduce readability and negatively impact accessibility for some users.
  • Professionalism: The practice makes emails look unprofessional and potentially deceptive.
  • Lack of Proven Benefits: There is no evidence to suggest that using 'O' instead of '0' improves email performance.

Key considerations

  • A/B Testing: While generally discouraged, A/B testing can be used to assess the specific impact on deliverability and engagement.
  • Target Audience: Consider whether the target audience might perceive the substitution as creative or confusing.
  • Brand Image: Evaluate whether the substitution aligns with the brand's overall image and communication style.
  • Accessibility: Ensure email design and practices adhere to accessibility standards for all users.
Marketer view

Email marketer from MarketingProfs advises against using letter substitutions in subject lines. They suggest that while creativity is important, clarity and professionalism should take precedence to maintain credibility.

June 2023 - MarketingProfs
Marketer view

Marketer from Email Geeks suggests testing the approach and notes that organizations sometimes follow outdated practices without proven benefits.

January 2022 - Email Geeks
Marketer view

Email marketer from Reddit believes that using 'O' instead of '0' might be a bad idea, as it could trigger spam filters and potentially harm deliverability. They suggest A/B testing to see if it affects open rates or spam scores.

January 2024 - Reddit
Marketer view

Email marketer from Email Marketing Tips Blog shares that while creative approaches can sometimes grab attention, substituting numbers with letters can make your email look unprofessional or even like spam. It is a risky approach with little potential benefit.

September 2021 - Email Marketing Tips Blog
Marketer view

Email marketer from EmailGeeks Community Forum strongly discourages replacing numbers with letters in email subject lines. They indicate that it can make the email look less professional and potentially be flagged as spam by various email providers.

August 2023 - EmailGeeks Community Forum
Marketer view

Email marketer from Marketing Forum recommends against using 'O' instead of '0.' They mention that it can impact readability and might annoy subscribers. Clear communication is key, and such substitutions can create confusion.

October 2021 - Marketing Forum
Marketer view

Marketer from Email Geeks questions the consistency of the letter O's appearance across different devices and asks about extending the practice to the email body.

January 2024 - Email Geeks
Marketer view

Email marketer from LinkedIn suggests that any perceived aesthetic benefit is outweighed by potential deliverability problems. The small visual change isn't worth risking being flagged as spam.

April 2024 - LinkedIn
Marketer view

Email marketer from Email Marketing Institute emphasizes that deliverability should be a top priority. Using 'O' instead of '0' might negatively affect sender reputation and email placement in inboxes.

October 2021 - Email Marketing Institute
Marketer view

Email marketer from Quora notes that while individual results can vary, most experts agree that such character substitutions can be detrimental. It's seen as a shortcut and can diminish trust with recipients.

September 2023 - Quora
Marketer view

Marketer from Email Geeks shares concerns about accessibility when using the letter O instead of the number 0 in subject lines.

May 2022 - Email Geeks

What the experts say
1Expert opinion

Laura Atkins, an expert from Word to the Wise, advises against using character substitutions like 'O' instead of '0' in email subject lines. She emphasizes that such practices make emails look unprofessional, trigger spam filters, and harm sender reputation because they often signal an attempt to deceive sophisticated email filters.

Key opinions

  • Negative Perception: Character substitutions make emails look unprofessional.
  • Spam Trigger: Such substitutions can trigger spam filters.
  • Reputation Harm: The practice damages sender reputation by suggesting deceptive intent.

Key considerations

  • Professionalism: Maintain a professional appearance in email communications.
  • Deliverability: Avoid practices that could negatively impact email deliverability.
  • Sender Reputation: Protect sender reputation by avoiding deceptive or questionable tactics.
Expert view

Expert from Word to the Wise, Laura Atkins, explains that character substitutions like using 'O' instead of '0' are a bad idea because they make an email look unprofessional and trigger spam filters. She notes that while email filters are becoming more sophisticated, these kinds of substitutions are often a sign that the sender is trying to hide something, therefore harming sender reputation.

February 2023 - Word to the Wise

What the documentation says
4Technical articles

Email deliverability documentation from Spamhaus, Google Support, Microsoft Support, and Mailchimp Resources all advise against using character replacements, such as 'O' instead of '0', in email subject lines. They emphasize that modern spam filters detect these tactics, which can negatively affect sender reputation, user trust, and deliverability, potentially leading to emails being marked as junk. Authenticity and clear communication are key for successful email marketing.

Key findings

  • Spam Detection: Modern spam filters are capable of detecting character replacements.
  • Negative Reputation Impact: Character replacements can negatively affect sender reputation.
  • Reduced Trust: Using such tactics reduces user trust.
  • Policy Violations: These practices violate anti-spam policies of major email providers.

Key considerations

  • Authenticity: Prioritize authentic and genuine communication.
  • Clear Communication: Focus on clear and straightforward messaging.
  • Policy Compliance: Adhere to anti-spam policies of email providers.
  • Reputation Management: Actively manage and protect sender reputation.
Technical article

Documentation from Google Support emphasizes the importance of clear and genuine communication. Using character replacements can be seen as deceptive, impacting user trust and deliverability to Gmail accounts.

July 2022 - Google Support
Technical article

Documentation from Microsoft Support warns against using deceptive techniques, including character substitutions. These practices violate their anti-spam policies and can result in emails being marked as junk.

March 2025 - Microsoft Support
Technical article

Documentation from Mailchimp Resources recommends avoiding character replacements in subject lines as it can trigger spam filters and negatively impact the overall performance of email campaigns. Authenticity is key for successful email marketing.

October 2021 - Mailchimp Resources
Technical article

Documentation from Spamhaus advises avoiding tricks like replacing characters (e.g., '0' with 'O') to bypass spam filters, as modern filters are sophisticated enough to detect these tactics, and such attempts can negatively affect sender reputation.

June 2021 - Spamhaus.org