What words and practices should be avoided in email subject lines?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 Jun 2025
Updated 15 Aug 2025
7 min read
Email subject lines are the gateway to your message. They're the first thing recipients see, and often, the only thing that determines whether an email gets opened, or sent straight to the junk folder. Crafting effective subject lines is a delicate balance, requiring an understanding of what compels people to click and, equally important, what triggers spam filters or causes recipients to delete your email unread.
The digital landscape is constantly evolving, and so are the algorithms mail servers use to identify unwanted email. What might have worked a few years ago could now land you on a blocklist (or blacklist, if you prefer that term). This means staying informed about the words and practices that can negatively impact your email deliverability is crucial for anyone sending emails, whether for marketing, transactional, or personal communication.
Avoiding certain elements in your subject lines can significantly improve your open rates and ensure your messages reach the intended inbox. It's not just about avoiding 'spammy' words, but also about cultivating practices that build trust and engagement with your audience.
Words that trigger spam filters
Spam filters are sophisticated, but they still flag certain words and phrases that are historically associated with unsolicited commercial email. While the effectiveness of these individual words as standalone triggers has diminished over time, using too many, or combining them with other suspicious elements, can still negatively impact your email's journey to the inbox. It is worth understanding that spam trigger words still relevant.
These words often relate to false promises, urgency, finance, or questionable products. For instance, words like 'free,' 'win,' 'cash,' 'discount,' 'guarantee,' and 'offer' are frequently scrutinized. While 'free' isn't an automatic ticket to the spam folder, its overuse, especially in combination with other problematic practices, can contribute to poor deliverability. You can learn more about how using the word 'Free' affects deliverability.
It's important to remember that context matters. A subject line like "Your Free trial is ending soon" is less likely to be flagged than "Claim your FREE cash prize now!!!" The latter is a clear indicator of spam, signaling manipulative tactics often used to bypass filters. A comprehensive list of spam trigger words can be found in various resources.
Common spam phrases to avoid
Financial offers: 'Extra income', 'double your money', 'work from home', 'credit card offers'.
Urgency and scarcity: 'Act now', 'limited time offer', 'expires today', 'don't miss out'.
Questionable content: Phrases related to health supplements, adult content, or illegal activities.
Deceptive tactics and misleading practices
Beyond specific words, certain practices in subject lines are designed to mislead or trick recipients, which is a significant red flag for email service providers. Deceptive subject lines not only harm your deliverability but also erode trust with your audience. This can lead to higher complaint rates, which will severely damage your sender reputation and lead to more of your emails being blocked (or blacklisted).
One common deceptive tactic is using fictitious reply or forward indicators, such as 'Re:' or 'Fwd:' when the email is not actually a reply or a forwarded message. This attempts to trick the recipient into thinking it's part of an ongoing conversation. While it might get an initial open, it quickly leads to frustration and marks as spam, or even unsubscribes.
Another practice to avoid is misleading content promises. The subject line should accurately reflect the content of the email. Promising something in the subject line that isn't delivered in the email body will quickly diminish trust and lead to negative engagement. This is not just about avoiding spammy phrases, but also about ethical communication. Misleading subject lines are a key reason emails go to spam.
Practices to avoid
Using 'Re:' or 'Fwd:': Deceptively implies an ongoing conversation.
Vague or clickbait headlines: Doesn't accurately represent email content, leading to disappointment.
Excessive personalization: Overdoing it can feel creepy or automated.
Recommended practices
Clear and concise: Straightforwardly state the email's purpose.
Honest representation: Subject line matches the email body's content.
Meaningful personalization: Use only when it adds genuine value to the recipient.
Formatting and stylistic blunders
The way your subject line looks can also influence whether your email lands in the inbox or the spam folder. Certain stylistic choices are frequently used by spammers and are therefore viewed with suspicion by mail servers. One of the most common is the excessive use of capitalization (all caps). Subject lines like "OPEN NOW FOR A HUGE DISCOUNT!" not only scream spam but can also appear aggressive or desperate to recipients. This immediately raises a red flag for both human readers and automated filters.
Similarly, overuse of punctuation marks, particularly exclamation points (e.g., "Limited Time Offer!!!"), can trigger spam filters and annoy recipients. While a single exclamation point can convey excitement, multiple ones are a clear indication of spam. The same applies to excessive dollar signs ($$$) or other special characters used to draw attention in an unnatural way. For example, using a dollar sign can cause issues.
Subject line length is also a factor. While there isn't a strict rule, overly long subject lines tend to get truncated on mobile devices, preventing recipients from seeing the full message. This can reduce engagement and open rates. Generally, keeping your subject lines concise (around 40-50 characters or 7-9 words) is a good practice for optimal display across various email clients and devices. Mailchimp recommends keeping them under 60 characters.
Finally, avoid using emojis excessively, especially if they are not relevant or if your audience isn't accustomed to them. While a single, relevant emoji can sometimes boost engagement, a string of them can look unprofessional and trigger spam filters. For more on this, consider if all-emoji subject lines hurt deliverability.
Element
What to avoid
Why to avoid it
Capitalization
ALL CAPS (e.g., "BIG SALE!")
Looks like spam, aggressive, hard to read, triggers filters.
Punctuation
Excessive exclamation/question marks (e.g., "Win!!!???")
Spam indicator, unprofessional, decreases trust.
Special characters
Strings of symbols (e.g., "$$$ FREE $$$")
Often associated with spam, can break display on some clients.
Length
Overly long subject lines (more than 50-60 characters)
Truncated on mobile, reduces readability and impact.
Final thoughts on crafting subject lines
In conclusion, avoiding certain words and practices in email subject lines is not just about dodging spam filters. While understanding how spam words affect deliverability is important, it's also about building and maintaining trust with your audience. Deceptive tactics, over-the-top formatting, and misleading language can quickly damage your sender reputation and lead to poor engagement.
The key is to be clear, concise, and honest in your subject lines. Focus on delivering value and accurately representing the content within the email. A well-crafted subject line is one that sets appropriate expectations and encourages opens based on genuine interest, not manipulation. Prioritize your recipients' experience, and your deliverability will naturally improve.
Regularly review your email metrics, including open rates and complaint rates, to understand how your subject lines are performing. Adapt your strategy based on what resonates with your audience and what keeps your emails out of the junk folder. This proactive approach ensures your messages consistently reach their destination and achieve their purpose.
Views from the trenches
Best practices
Keep subject lines concise and to the point, aiming for around 40-50 characters to ensure full visibility on mobile devices.
Always accurately reflect the email's content in the subject line to build trust and avoid recipient disappointment.
Personalize subject lines thoughtfully, using recipient data only when it adds genuine value and relevance.
Common pitfalls
Using all capital letters in the subject line, as it appears aggressive and is often flagged by spam filters.
Excessive use of exclamation points or special characters, which can make emails look unprofessional and spammy.
Employing deceptive tactics like 'Re:' or 'Fwd:' when the email is not an actual reply or forward.
Expert tips
Focus on the benefit to the recipient in your subject line, clearly communicating what they will gain by opening the email.
Create a sense of gentle urgency or curiosity without resorting to aggressive or misleading language.
Monitor your open rates and spam complaint rates closely to identify subject line issues early and make necessary adjustments.
Marketer view
A marketer from Email Geeks says that you should avoid using 'FREE' or excessive exclamation points like '!!!' in subject lines, based on information they found online. It's perceived as a rule to keep emails out of spam.
2020-06-03 - Email Geeks
Expert view
An expert from Email Geeks clarified that the idea of avoiding 'FREE' or '!!!' in subject lines, as well as the notion that AOL blocks emails with pink backgrounds, are decades-old folklore and largely incorrect today. Spam filtering is far more sophisticated now.
