Most of the time, spam filters work amazingly, saving us from an inundation of scandalous, fraudulent, or other junk emails. But sometimes they fail, diverting legitimate emails you need to your spam folder.

Spam filters can mistakenly flag emails that aren’t spam at all – sometimes, even important ones. False positives are more likely when you write someone for the first time, or send a message to someone on a different domain or email provider. This is problematic, as such emails range from job applications to sales outreach to following up with someone after meeting them at a conference. (A current Boomerang employee’s application almost went unnoticed when it was flagged as spam, so we’ve experienced the dangers of false positives firsthand!)

We’ve written a lot on how to get more responses to your emails, but a message that lands in a spam folder is unlikely to be seen, let alone replied to. So we researched what exactly spam filters look at when deciding if a message is spam or not. Some rules make sense, others are wacky, but all of them can penalize your message. Without further ado, here are 16 ways you could inadvertently trigger a spam filter, based on rules used by SpamAssassin, a popular open source spam filter:

1. You are both excited! And have a question?

A subject line that contains both a question mark and an exclamation mark can trigger a spam filter rule, regardless of context. So you might be best off curtailing your excitement, and not draft a very important email with the subject line “Did you get the proposal? Let me know!” (Source)

2. You need an urgent reply or assistance (and ask for it.)

This might be the worst rule to accidentally trigger when you genuinely need an urgent reply. Including a phrase like “urgent reply needed” in your email can cause spam filters to apply a penalty to your email. So ironically, your self-proclaimed “urgent” emails might end up less likely to be seen than other (non-urgent) ones you send. (Source)

3. You keep it more than 💯 (% in your subject line.)

Including a percentage over 100% in your subject line is a trigger for spam filters. If you’ve spent hours penning the most motivational of emails to your team, don’t doom yourself with a subject like “Let’s put in 110% this quarter” or they may never get it. (Source)

4. You like to communicate with images, gifs, or memes.

GIPHY integration is seemingly everywhere, memes are so mainstream that they may have influenced the last American presidential election, and conventional wisdom suggests a photo is worth a thousand words. But emails that are heavy in images and light in text are more likely to be flagged as spam! The higher the ratio of images to text in your email, the higher your chances of triggering a spam filter. (Source)

5. You are an excitable Oprah fan.

Oprah is great. But for some reason, being too excited about Oprah in your email can trigger spam filters:

“I like Oprah.” is fine. ✅

“I like Oprah?” is also fine. ✅

“I like Oprah!” is apparently a possible indicator of spam. ❌

Turns out we’re also not the first ones to be amused by this rule, or its shorthand name. (Source)

6. You’re too formal with your email openings.

The moment your email contains “Dear Sir”, your email has an increased chance of being marked as spam. Using “Dear” is fine in most cases though, so while “Dear Sir Ian McKellen” will trigger this rule, “Dear Gandalf” is fine! (Source)

7. You’re too much of a bro with your email openings

For some reason, some spam filters have a specific rule that looks to see if an email contains the phrase “Hey bro,” So be wary, fratstars — those rush event invites could be flagged as spam. Luckily, there are a lot of email openings that span the spectrum between “Hey bro” and “Dear sir”, so hopefully you can find one that fits your email. (Source)

8. You make imperfect edits to your HTML newsletter or email.

If you work with HTML email templates, you might accidentally leave an extra closing tag around or forget to include one. The good news is most modern browsers and email clients do a pretty good job about rendering (or ignoring) superfluous or missing HTML tags. The bad news is spam filters aren’t as forgiving, and having malformed HTML in your email can trigger a spam penalty. (Source)

9. You didn’t include an accurate plain-text version of that killer HTML email you put together.

Anyone who sends marketing emails knows it can be a pain to get the perfect HTML email together that looks great across devices and email clients. But all that work can be for naught if you don’t include an equivalent plain-text version of your email. (Sidenote, Mailchimp has a great tool that will do it for you.) If you don’t include a plain-text version of your email, or if you do but it differs noticeably from your HTML email, you can be penalized. (Source)

10. You mark your email as having “High Importance” in Outlook.

You hope by marking an email as high priority, that the resulting red exclamation mark next to the message will make your recipient open it sooner. But spammers hope for the same thing. Sending an email with high priority can end up hurting your email’s deliverability, as spam filters see it as a potential sign your message is spam. (Source)

11. You use a free webmail provider for your business email.

Setting up a Google Apps email with your domain name costs money, so you’re using a free Gmail address for your business. Not a bad plan. But you should know that some domains use a SpamAssassin plugin to (slightly) penalize emails from free email domains like gmail.com, yahoo.com, or hotmail.com.

As a longtime Gmail user, the chance of a small penalty because I use a free email provider isn’t something that worries me, nor should it concern most users. But if you’re frequently writing cold outreach/marketing emails from a free webmail address, it could be enough to push your email over the edge into spam territory for some domains. (Source)

12. You email people asking if they want or have something.

“Do you want to discuss yesterday’s interview?” seems like a pretty relevant subject line. “Do you have tomorrow’s agenda?” is a reasonable ask as well. Unfortunately, a lot of spam messages have subjects that start off with phrases like “Do you want” and “Do you have”, so starting your subject off like this is another easy way to accidentally trigger a spam filter. (Source)

13. You’re just trying to help (in your subject line.)

Perhaps you’re at the bottom of the totem pole in the corporate world looking to earn your stripes. “I will help with the proposal!” you write as you enthusiastically offer help with business development. Or maybe you’ve found the killer paper for a friend to reference in their thesis literature review. “Hope this will help!” you exclaim. Unfortunately, no matter your intentions, including the phrase “will help” anywhere in your email’s subject is reason enough for a spam filter to penalize your message. (Source)

14. You refer to your friend as “friend.”

This is a spam rule that even the White House triggered at one point: opening your email with “Dear Friend.” The logic here is that if you’re really writing an email to a friend, you probably won’t start with “Dear Friend.” Instead, more generic (or spammy) emails are likely to open in such a fashion. (Source)

15. Your last name is Cash.

Cash is a pretty common surname. Unfortunately anyone with this last name could start with a strike against their email. Having the word “cash” in your from field increases your message’s spam score.

Sure, it makes sense to penalize a message from “Free Cash Now” <getfreecashnow@spammers.biz>, but there’s over 30,000 people with the last name Cash just in the United States who also get penalized. Maybe Johnny Cash did reply to that fan mail I sent him, and my spam filter just made me miss it… (Source)

16. You’ve managed to get an apostrophe in your email address.

The luck of the Irish is typically pretty bad with email addresses: most email providers won’t let you create an email address with an apostrophe in it, even if your last name is O’Malley or O’Connor. That said, it’s still possible to create an email address with an apostrophe (I’ve personally seen such addresses on a few occasions) but it’s still not a good idea. Simply having an apostrophe in your email address can trigger spam filters. (Source)

Final Thoughts: There’s a lot more to know if you want a reply.

At the end of the day, getting your email to show up in someone’s Inbox is just part of the battle. There are still messages that, while not seen as spam, aren’t likely going to get a response. Luckily, there’s a tool for helping you write more effective emails called Respondable, which uses AI to help guide you toward an email that is more likely to get a reply.

Add Respondable for GmailGet Respondable for Outlook

In conclusion, don’t be too excited about Oprah, don’t “hey bro” anyone, and with any luck, your messages will show up in your recipients’ Inboxes, not their spam folders.