What is an ISP Spam Filter & How Does It Work?

An ISP spam filter will absolutely affect your cold email deliverability as a salesperson. Learn how they work, how to stay out of spam, and how an email warm up tool can help you!
We all know what spam is: the annoying telemarketing-style emails you receive that your emails went to spam because they might also have a more sinister purpose.

Sometimes, though, the spam filter messes up and sends the wrong emails to the spam box, and you then have to dig through it to find them.

For salespeople, avoiding hitting that filter is a top priority.

Let’s learn more about the ISP spam filter, how it works, and how to avoid it!

What is The ISP Spam Filter?

An ISP spam filter is a system Internet Service Providers (ISPs) use to prevent unwanted and unsolicited emails from reaching their customers' inboxes.

These spam filters work by examining the content and characteristics of each email message as it passes through the ISP's network. The filter will analyze various elements of the message, such as the sender's address, the message content, and any attached files, to determine whether the message is likely to be spam.

If the filter identifies an email as spam, it may either block the message entirely or move it to a separate folder designated for spam or junk mail. Some ISP spam filters also allow users to customize the filter's settings, allowing them to block specific senders or types of messages.

ISP spam filters are important in preventing spam and protecting users from potentially harmful or unwanted emails.

How do ISP Spam Filters Work?

As mentioned above, ISP Spam Filters utilize a variety of tools and processes to identify incoming emails as possible spam.

They look for spam-esque characteristics, calculate the likelihood of the email being malicious or spam, and then weigh the total risk against a predetermined threshold. If it is too high, the email is flagged as spam.

Typically, the tools analyze:

Email Headers

Focusing on suspicious email addresses or ones that have not gone through a checklist for deliverability, or aspects such as misspellings.

ISPs may use various techniques to analyze the email headers, such as checking for suspicious or inconsistent information, such as mismatched sender and recipient domains, missing or invalid email authentication, or unusual message content, such as excessive use of capital letters, HTML tags, or known spam keywords.

They may also use machine learning algorithms to learn from past spam emails and identify patterns that indicate spam.

Email Content

Filters will check for file names, bolded or italicized or all caps, text size, images, and attachments. They may use algorithms to check for suspicious patterns, such as excessive use of certain words, phrases, or characters, or missing elements typical of legitimate emails, such as a salutation or a signature.

Also, they use machine learning to learn from past spam emails and identify new patterns that indicate spam. The goal is to prevent unwanted and potentially harmful emails from reaching their users' inboxes while ensuring that legitimate emails are delivered.

Blacklist Filters

ISP's spam filters use blacklist filters to identify and block emails from known spammers.

They compare the email's IP address and domain name to those on existing blacklists. If the email matches an entry on the blacklist, it will be marked as spam and either blocked or sent to the user's spam folder.

To avoid being blacklisted, email senders should regularly check their IP addresses and domain names against existing blacklists using specialized tools.

If they are listed, they should address the issues that caused them to be blacklisted, such as identifying and removing compromised accounts or addressing email deliverability issues.


ISP's spam filters also consider permissions when filtering emails.

Email senders must obtain permission from recipients to send them emails. If a recipient has not given permission to receive emails from a particular sender, the email may be marked as spam.

ISP's spam filters use a variety of techniques to determine whether an email has been sent with the recipient's permission. They may check for the presence of opt-in information, such as a confirmation link, in the email header or body - they may also compare the email's sender information and content to what the recipient has previously agreed to receive from the sender.

If the email is found to be in violation of the recipient's permissions, it may be marked as spam and either blocked or sent to the user's spam folder. By obtaining explicit permission from recipients and respecting their preferences, email senders can help ensure their emails are delivered to their intended recipients' inboxes.

Rule-Based Filtering

ISP's spam filters also use rule-based filtering to identify and block spam emails. These filters are set up by algorithms to identify spam based on specific rules, such as the presence of certain words or phrases commonly found in spam emails.

ISPs may use a combination of pre-defined rules and customized rules to identify spam. For example, they may set rules to check for particular phrases or patterns in the email's subject line or body, such as "buy now" or "act fast." They may also set rules to identify emails with suspicious attachments or links.

If an email matches one or more pre-defined rules, it will be marked as spam and either blocked or sent to the user's spam folder.

By avoiding spam-triggering phrases and following email best practices, email senders can help ensure their emails are not incorrectly marked as spam by rule-based filters.

What Are Spam Filters For?

While the above may seem a complicated formula for salespeople looking to provide value for prospects, spam filters serve a specific purpose.

And that is security.

ISPs utilize spam filters to protect and safeguard the information of their users by preventing the spread of malware.

At their most basic level, yes, they prevent emails from reaching prospects, but at the end of the day, spam filters can be the difference between a data leak or a company-wide virus.
Improve your email deliverability with Unfiltered.ai

Typical Email Spam Formats

Phishing & Fake Advertising

Phishing emails are spammers impersonating a real person or business, attempting to convince their target that they have a valuable, legitimate offer for them.

Their main goal is obtaining personal details, such as usernames, passwords, bank details, etc.

Of course, when this information is being asked for, the person may have already been clued in.

But the beginning of these attempts is very similar to cold emailing, which may initially make it difficult for salespeople to stand out as different and legitimate.

Baiting & Hoaxes

Similar to the above, in that the spammer is passing themselves off as legitimate, it concerns anyone using email marketing to reach out to potential customers.

Spammers will convince those on the receiving end that they have won a reward or are eligible for an exclusive offer they must claim and that it is time-sensitive. Otherwise, they miss out.

With this, they can obtain the personal data of those who fall for their trick.

Malicious Content

The final option is the link or attachment with a hidden virus, malware, or ransomware – the infamous Trojan Horse.

Once the link has been clicked or the attachment opened, the receiver’s computer has fallen prey to the spammer, and who knows what may have been leaked.

How to Prevent Being Flagged as Spam

Salespeople and marketers are not looking to obtain personal data or infect their prospect's computers with a virus.

Our main goal is to offer actual value to prospects that fit our Ideal Customer Profile and see if they may be interested in meeting to discuss it further.

But because our techniques are similar to those of ill-intentioned spammers, it is worth going through a checklist.

After all, these ISP spam filters are just trying to protect their users.

Do a Weekly Check of Your Email Deliverability

Use a tool such as GlockApps to determine the status of your domain’s email deliverability on a weekly basis and discover your sender's reputation.

See if you are being sent to spam more often than not.

If you are, use a warm-up tool such as Unfiltered to help legitimize your domain and demonstrate that you are not a spammer.

Slow-Down Your Email-Sending

In addition to using an email warm-up tool, it may be a good idea to slow down on how many emails you send hourly, daily, or weekly.

You will have fewer one-sided conversations and email chains, which adds further credibility to your email address and domain in the eyes of spam filters.

Ensure Your DNS Setting Are Set

Your DMARC, SPF, and DKIM need to be set up correctly for you to engage in best email practices.

Without them, ISPs will evaluate your sender reputation as low and send you directly to spam.

Use a Spam-Trigger Word Checker

Write your own copy, but always run it through a spam-trigger word checker before sending it out.

You never know or may not realize how much of your wording may be flagged as spam. If you don't do this, the entirety of your copy may never even be seen by your prospect!

Ensure You Are Following Cold Email Best Practices

Cold email best practices exist for a reason, and following them means it is less likely that your email will be flagged as spam.

Essentially, you want always to personalize your email and approach your prospect with a well-researched, value-focused offer. Have a clear call to action and an attractive subject line as well.


ISP spam filters have an important job: protecting their users from being attacked by malware for data leaks.

Many of the strategies spammers use are a little too similar to cold emailing, which means salespeople have to ensure they’re offering their prospects the best possible value in their messaging.

Always check your email deliverability. Sometimes it’s not the messaging but the technicalities of email that are holding you back.

And remember – whenever you need to warm up your domain, Unfiltered is here for you.
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