Does using 'O' instead of '0' directly trigger spam filters?

Using 'O' instead of '0' can indeed trigger spam filters. These filters look for unusual character substitutions, often associated with spam or phishing attempts to bypass detection. Even if your content is legitimate, this practice can increase your spam score and negatively affect your domain reputation .

How does this practice affect email accessibility?

Yes, it significantly impacts accessibility. Screen readers, used by visually impaired individuals, interpret 'O' as the letter 'oh' and '0' as the number 'zero'. This can cause confusion and misinterpretation of your subject line's meaning, making your email less accessible to a portion of your audience.

Can substituting characters impact my email metrics?

Yes, it can. First, a suspicious or confusing subject line may lead to lower open rates. Second, analytics tools might misinterpret or fail to parse the substituted characters, leading to inaccurate data on email opens and other metrics. This makes it harder to assess campaign performance effectively.

Are there any benefits to using 'O' instead of '0'?

No, there are generally no proven benefits to using 'O' instead of '0' in email subject lines. Any perceived aesthetic uniqueness is typically outweighed by the negative impacts on deliverability, accessibility, and user perception. There's no evidence to suggest it improves open or click-through rates .

What are the general best practices for including numbers in subject lines?

For optimal results, keep subject lines clear, concise, and relevant to the email content. Use standard characters and punctuation. Numbers can be effective for lists, dates, or discounts, but always use the numerical digits (e.g., '10% off' not '1O% off'). Avoid practices that could be mistaken for spam, such as excessive capitalization or unusual symbols.

Is using the letter O instead of the number 0 in email subject lines a good practice? - Sender reputation - Email deliverability - Knowledge base

A client recently asked about using the letter 'O' instead of the number '0' in email subject lines, for instance, writing O3/1O/23 instead of 03/10/23. My initial thought was that this practice might negatively impact deliverability, especially given how spam and phishing emails often use lookalike characters to deceive recipients.

The reasoning behind such a request often stems from a desire for a unique aesthetic or to stand out in a crowded inbox. However, when it comes to email deliverability and user experience, seemingly minor stylistic choices can have significant, unintended consequences. It is crucial to evaluate these practices from technical, accessibility, and human perception standpoints.

I've explored this topic to understand the potential risks and benefits, if any, of using the letter 'O' in place of the number '0'. My findings suggest that while the aesthetic might appeal to some, the downsides for email deliverability and overall effectiveness far outweigh any perceived advantages.

The perception paradox and user experience

One of the immediate concerns with substituting 'O' for '0' is the impact on user perception and experience. While the characters may look similar, particularly in certain fonts, they are fundamentally different. Most users expect numbers to be represented by numerical digits, and deviations can create confusion or a sense of unease. This subtle difference can make a subject line appear unprofessional or even suspicious to a discerning recipient.

Beyond aesthetics, there are significant accessibility implications. Screen readers, which are used by individuals with visual impairments, differentiate between letters and numbers. An 'O' will be read as the letter 'oh', while a '0' will be read as 'zero'. This can lead to misunderstandings and frustration for users relying on assistive technologies. Ensuring content is accessible is a critical aspect of modern email practices, as outlined in guidelines like Web Content Accessibility Guidelines (WCAG).

Furthermore, subject lines are displayed across a vast array of email clients, devices, and operating systems, each with its own font rendering. A character substitution that looks appealing in one font might look completely out of place or confusing in another. This inconsistency undermines any intended aesthetic benefit and can lead to a disjointed brand experience.

Using the letter 'O'

User experience: Can appear unprofessional, suspicious, or like a typo to recipients.
Accessibility: Screen readers interpret 'O' as a letter, not a number, causing confusion.
Readability: Varies greatly across different fonts and email clients, leading to inconsistencies.

Using the number '0'

User experience: Clear, conventional, and professional representation of numerical data.
Accessibility: Correctly interpreted by screen readers as a numerical digit.
Readability: Consistent rendering across various platforms, ensuring clarity.

Deliverability implications and spam filters

Perhaps the most critical aspect of this practice is its potential impact on email deliverability. Spam filters and blocklists (or blacklists) are constantly evolving, using sophisticated algorithms to identify and flag suspicious emails. One common tactic used by spammers and phishers is character substitution (sometimes called leetspeak) to bypass filters while still conveying their message to humans. By using 'O' for '0', you are inadvertently mimicking this behavior.

Even if your email content is legitimate, this kind of substitution can trigger a red flag with a spam filter. These filters look for patterns, and character substitutions are a known indicator of potentially malicious or unwanted mail. Over time, consistently using such practices could negatively affect your sender reputation, leading to lower inbox placement rates and increasing the likelihood of your emails landing in the spam folder or on an email blacklist or blocklist. This is why it's vital to know what to avoid in subject lines.

Mailbox providers like Mailchimp and Salesforce consistently advise against practices that can be misinterpreted as spam. Their recommendations focus on clarity and adherence to standard conventions to maximize inbox placement. Using standard numerical digits ensures that automated systems correctly process your subject lines, reducing the risk of misclassification as spam or phishing attempts.

Risks of character substitution

Using 'O' for '0' in subject lines can trigger spam filters due to its resemblance to techniques used by spammers and phishers. This practice can signal suspicious behavior to automated systems, potentially leading to lower inbox placement, increased spam complaints, and even contributing to your domain or IP being added to a blocklist (or blacklist). Prioritizing clear, standard characters helps maintain a positive sender reputation and ensures your emails reach the intended inbox.

Impact on engagement and metrics

The primary goal of a subject line is to encourage opens and engagement. If a subject line looks odd or unprofessional due to character substitution, it is likely to have a negative impact on your email open rates. Recipients might simply ignore or delete the email, perceiving it as spam or low-quality content. Numbers, when used correctly, can make subject lines more scannable and convey information quickly, but this benefit is lost with confusing character choices.

Moreover, internal analytics and reporting tools might not correctly interpret subject lines that use non-standard characters. This can lead to skewed data, making it difficult to accurately track the performance of your email campaigns. If your tracking systems expect numerical digits and encounter letters, it could lead to miscategorization or data loss, hindering your ability to optimize future efforts.

Ultimately, the effectiveness of your email marketing hinges on trust and clarity. Any practice that introduces ambiguity or reduces professionalism can deter recipients from opening your emails, regardless of how compelling the content inside might be. The potential for a slight aesthetic preference to compromise engagement and data integrity is simply not worth the risk.

Aspect	Using 'O' for '0' (e.g., O3/1O/23)	Using standard '0' (e.g., 03/10/23)
Readability	Can be confusing; may look like a typo or suspicious to recipients.	Clear and universally understood, aiding quick comprehension.
Spam Risk	Higher risk of triggering spam filters due to character substitution.	Lower spam risk, as it adheres to standard, expected patterns.
Engagement (Open Rates)	Potential for lower open rates if perceived as suspicious or unprofessional.	Higher potential for engagement due to clarity and professionalism.
Analytics & Tracking	May cause issues with parsing and accurate data collection.	Ensures accurate tracking and analysis of subject line performance.

Best practices for subject lines

To ensure optimal email deliverability and engagement, adhering to established best practices for subject lines is paramount. This means focusing on clarity, relevance, and professionalism. Subject lines should accurately reflect the email's content and be easily understood by both human recipients and automated filters.

Incorporating numbers in subject lines can be effective for conveying specific information, such as lists, dates, or discounts. When doing so, use standard numerical digits to maintain clarity and avoid triggering spam flags. For example, 5 New Features You'll Love is effective, whereas 5 New Features YOu'll L0ve would not be.

Regularly testing your subject lines through A/B tests can provide valuable insights into what resonates with your audience. However, these tests should be conducted within the bounds of standard, risk-averse practices. Experimenting with character substitutions, while seemingly innovative, introduces unnecessary deliverability risks that can compromise your entire email program. Prioritize a positive sender reputation and clear communication over risky aesthetic choices.

Email subject line examples

Good: "Your order #12345 has shipped!"
Good: "Last Day: Get 10% off your next purchase."
Bad: "YOur 0rder #12345 has shipped!"
Bad: "Last Day: Get 1O% off your next purchase."

Views from the trenches

Best practices

Always prioritize clarity and conventional formatting in subject lines to ensure readability.

Utilize standard numerical digits (0-9) for any numbers in your subject lines to avoid confusion.

Focus on creating subject lines that accurately reflect email content and manage subscriber expectations.

Conduct A/B testing on subject line elements, but avoid risky character substitutions.

Regularly monitor your email deliverability and sender reputation to catch potential issues early.

Common pitfalls

Using 'O' instead of '0' in subject lines can appear suspicious to both recipients and spam filters.

Ignoring accessibility considerations, which can alienate users relying on screen readers.

Sacrificing clarity for perceived aesthetic uniqueness, leading to lower engagement.

Failing to track email performance accurately due to inconsistent or unusual subject line formatting.

Adopting practices simply because others in the industry might have used them previously without clear evidence of benefit.

Expert tips

Email Geeks recommends focusing on universally understood characters to prevent misinterpretation by automated systems.

Email Geeks advises that inconsistencies in font rendering across devices can nullify any intended stylistic appeal.

Email Geeks suggests that character substitution can resemble tactics used in malicious email campaigns.

Email Geeks emphasizes that any perceived aesthetic gain from using 'O' for '0' does not outweigh the deliverability risks.

Email Geeks recommends a pragmatic approach, favoring robust deliverability over stylistic quirks in subject lines.

Expert view

Expert from Email Geeks says that using the letter 'O' instead of the number '0' in subject lines would likely damage the accessibility of emails.

2023-03-10 - Email Geeks

Expert view

Expert from Email Geeks states that client requests like this are definitely bizarre and should be questioned.

2023-03-10 - Email Geeks

The best path for subject lines

While the idea of using the letter 'O' instead of the number '0' in email subject lines might seem like a creative way to stand out, it introduces more risks than benefits. From a deliverability standpoint, it can flag your emails as suspicious to spam filters, potentially leading to lower inbox placement and harming your sender reputation. It also creates accessibility barriers for users who rely on screen readers and can lead to inconsistent rendering across different email clients.

Furthermore, such a practice can negatively impact user engagement. Recipients are accustomed to seeing standard numerical digits, and deviations can make your emails appear unprofessional or even malicious, reducing open and click-through rates. The slight aesthetic preference does not justify the potential damage to your email program's effectiveness.

My recommendation is to stick to conventional, clear, and accessible subject line practices. Use standard numerical digits (0, 1, 2, etc.) to ensure your messages are accurately interpreted by both email systems and human recipients. Prioritizing deliverability, accessibility, and a professional appearance will always yield better long-term results for your email marketing efforts.