Pardot Knowledge Base

Spam Filters and How to Avoid Them

Last Updated: Feb 23, 2017 | Print this Article

To avoid spam filters, you need to understand how they work. This article will explain how spam filters do their filtering, and will also give you plenty of tips and best practices to avoid ending up in the spam folder.

What is spam?

The simplest definition of spam is unsolicited bulk email. Unsolicited means that it's being sent without the recipient's consent. Bulk means it's being sent to a lot of people at once. Spam is such a problem that we have laws in place to try to control it. Because almost 90% of all email is considered spam, we also have spam filters. Pardot doesn't tolerate spammers, and we expect every client to follow our Permission-Based Marketing Policy.

How do spam filters work?

We've got good news and bad news. The bad news is that the specifics of how spam filters work is a jealously-guarded secret — they don't want spammers knowing how to beat them. The good news is that we do know some things about how they work so legitimate marketers can still get their emails into the inbox. Spam filters generally look at a few things when deciding what to do with emails: the sender's technical specifics, the email's content, and engagement.

Technical Specifics

Spam filters typically check for appropriate email authentication and your sending IP's reputation when deciding whether to deliver your emails or make them disappear into spam limbo. It's a really good idea to use email authentication and protect your sending reputation if you actually want to get your messages into inboxes.

Email Authentication

Do you have email authentication set up? The two most common authentication standards are SPF and DKIM. Every major ISP and most major spam filter providers check for one or both of these when deciding what to do with an email. If you're using Pardot to send emails, setting up your email authentication is critical to having good deliverability. See this article to learn more.

Sending IP Reputation

Spam filters look at your sending IP when deciding what to do with your emails. If your sending IP reputation get damaged, it can really hurt your deliverability. Our Email Reputation Team takes IP reputation really seriously here at Pardot because if someone gets blacklisted for being spammy, it can affect deliverability for other clients. You can maintain a good IP reputation by following our Permission-Based Marketing Policy, warming up your IP address, and having good list hygiene.


Here we're talking about content — the stuff that's actually in your email subject and body. It's easy to make a few missteps and have your  legitimate email trigger spam filters. Keep an eye out for these common mistakes, and avoid them where you can (protip: you can almost always avoid these mistakes).

Interactive Content

Including some kinds of 'interactive content' in your emails could cause your emails to be marked as spam. At Pardot, we've noticed that including the following in your email code could trigger spam filters:

  • JavaScript
  • RSS feeds
  • Forms

Messy Code

One of the most common causes of sloppy email HTML is copy-pasting from Microsoft Word. Please don't do that — when you copy-paste content from Word, it pulls in styling tags and all kinds of other stuff. If you must copy-paste, make sure you're using the Paste From Word button in the Email Flow Editor. Also, be sure to check your email source code for any messy HTML or other code before you send to your prospects.

Suspicious Formatting

Don't do suspicious things with your text formatting like:
  • using punctuation!!!??? in your subject line
  • d0ing w3ird th1ngs with letters and numbers
  • using too many different font colors
  • using large font sizes (anything greater than 10pt or 12pt)
Not only can these sorts of things trigger spam filters, they might also annoy your recipients into marking your emails as a spam.

Image-to-Text Ratio

Too many images (or too large of an image) compared to text in an HTML email can be considered spammy. Adding more text and/or reducing the number and size of images may help. Keep in mind that lots of email clients block images by default, so there's a good chance that even if your image-heavy emails make it into the inbox, your recipients don't see the images automatically. Also, make sure your images include alt tags. For more on this check out The Ultimate Guide to Styled ALT Text in Email from Litmus.


Avoid using URL shortners in your emails — they're frequently abused by spammers. Some shortened domains (including have been placed on widely-used block lists which means that emails containing these links will be blocked by many spam filters. It’s also a best practice to limit the number of different domains linked in an email. Pardot automatically rewrites links in your emails (as long as they start with “http://”) using your vanity CNAME, so these issues shouldn’t cause problems unless a shortened URL is not prefixed with “http://” or the link is placed in a blockquote tag.

Missing Information

According to CAN-SPAM, your marketing emails must contain two things: your physical address and a way for recipients to opt-out of future emails. This is the law. If your marketing emails are missing an unsubscribe/email preference center link or a physical address, they'll trigger spam filters. Pardot automatically includes your physical address and unsubscribe link on your marketing emails, so you should be good to go.

List Management

List management is important. If you’re sending to unengaged prospects who aren’t opening and clicking your emails, mail servers are going to assume you're a spammy sender. Why, you ask? Because email providers and ISPs are going to notice that your emails are being ignored by recipients and assume you're sending spam. So, what can you do to look good to mail servers?

  1. Have a sunset policy for unengaged prospects. After sending a set number of emails to a prospect with no engagement, remove them from your mailing lists.
  2. Don't send to old lists! Prospects on these lists probably won't remember who you are, so they’re likely to mark your message as spam (see Abuse Reports below for more on this). This is fatal to your email deliverability. You could also end up sending to a spamtrap, which could get you blacklisted.

Abuse Reports

Abuse reports happen when someone marks an email as spam in their email client. Abuse reports are super serious, and even a relatively small number of spam complaints can get emails from your sending IP blocked.

Why do people report legitimate emails as spam?

There are a few reasons people may mark your emails as spam:

  • Timing: you took too long between collecting the email address and sending an email. your recipients might not remember you.
  • Consent: your email lists aren't 100% opted-in.
  • Volume: you're sending too many emails, too often.
  • In-person signups: your prospects signed up at a tradeshow or other event and don't remember opting in.
  • Partner lists: you're sending to a partner or co-marketing list, and recipients don't connect the dots between your emails and the list they initially signed up for.

How do I prevent false spam complaints?

You can prevent false spam complaints by doing the opposite of the things listed above:

  • Email your prospects soon after they opt in.
  • Use a confirmed opt-in process.
  • Never ever ever use purchased lists.
  • If you email prospects who signed up at a tradeshow or other event, remind them of how you got their email address.
  • Ask subscribers how often they want to receive your emails.
  • Ask for permission to email and document it. For example, use a confirmed opt-in process. If requesting permission to email over the phone, send a one-to-one email after the call, thanking the prospect for opting in.
  • Never buy lists. This is so important it bears repeating.

Why is my email going to spam?

You can take some guesses about why your emails are going to the spam folder depending on where you're sending the email.


Gmail has the toughest spam filtering in the industry. We don't have any specific information about why Gmail passes or fails emails, but we've noticed that these things tend to trigger Gmail's spam filter:

  • Bad SPF statement: make sure your SPF is set up correctly.
  • Missing unsubscribe link: Pardot automatically includes this on your list emails, so you're covered.
  • Missing physical address: Pardot automatically includes this on stock email templates. You'll need to add it to email templates you create from scratch.
  • Your IP's reputation: Gmail takes IP reputation pretty seriously, so make sure yours is good.
A simple way to test your Gmail deliverability is to do the following:
  1. Create a Gmail account and associate the email address with test prospect.
  2. Add the prospect to a email test list.
  3. Send your email to your test list.
  4. If your email goes to the spam folder in the test Gmail account, Gmail will usually provide a reason it went to spam. See this documentation for more help.
Use the information that Gmail provides to make necessary adjustments before sending a live email to your prospects.


If you sent a test email to internal email addresses and they ended up in the spam folder or you got a hard bounce, you need to make sure your sending IPs are whitelisted correctly in your internal spam filter. Get with your IT department and double-check your whitelist. See this article for more.

A Specific ISP

A few things could cause a specific ISP, like Comcast or Hotmail, to junk your emails:

  •  A bad IP reputation will get your email junked. Protect your IP reputation!
  • Not having a sunset policy in place for unengaged prospects leads to bad engagement metrics, which makes your email look unwanted and spammy.
  • Not segmenting out your prospects can make you look spammy. For example, if you're sending tons of emails to Comcast email addresses in one day, they're going to assume you're spamming. Segment out your most active prospects and send to them first — this helps show ISPs that your emails are legitimate and wanted.
  Not what you're looking for? Check out these other articles:
Permission-Based Marketing Policy
Email Compliance and Deliverability Overview
List Compliance FAQ

Need more? Start a conversation with other Pardot users in our Success Community