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

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We all know what spam is: the annoying telemarketing-style emails you receive that your email shoves into the spam box because they might also have a more sinister purpose.

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

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

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

What Is ISP Spam?

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ISP stands for Internet Service Providers.

Think of Hotmail, Gmail, Outlook, AOL, etc.

These providers have a duty to their customers to protect them from dangerous or unwanted emails. To do this, they use a variety of tools that process and filter all incoming emails.

It is through the use of these tools that you end up having important emails sent to spam.

ISP Spam, therefore, means any unwanted content directed at the mailboxes of their users.

And these are the filters that salespeople have to be mindful of when cold emailing prospects.

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.

Email content – Filters will check for file names, bolded or italicized or all caps, text size, images, and attachments.

Blacklist filters – they will compare IP addresses and domain names to those of senders on existing blacklists, as they can cross-examine. Utilizing tools to warm up and determine if you’re on a blacklist is very important when setting up a new domain.

Permissions – filters will compare to what the recipient has previously agreed to receive from the sender. Anything that has not been agreed upon will go to spam.

Rule-based filtering – these are the filters set up by algorithms to identify spam, such as particular phrases. There are particular words or phrases that spam checkers will help you recognize and avoid.

What Are Spam Filters for?

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

Typical Email Spam Formats

For anyone sending out cold emails or engaging in email marketing, it is a good idea to be educated on what malicious spam emails look like.

There are 3 typical formats:
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Phishing and 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 to obtain personal details, such as usernames, passwords, bank details, and so on.

Of course, by the time this kind of 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 and hoaxes

Similar to the above, in that the spammer is passing themselves off as legitimate, it is concerning for 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 are able to obtain the personal data of those who fall for their trick.

Malicious content

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

Once the link has been clicked on 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 computer’s with a virus.

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

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

After all, these ISP spam filters are just trying to protect their users.
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Do a weekly check of your email domain’s 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 reputation.

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

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

Know if you have to slow down on sending emails

In addition to using a warm up tool, it may be a good idea to slow down on how many emails you are sending 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.

Make sure your DNS security settings are set up

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 word checker

Write your own copy, but always run it through a spam 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 seen by your prospect!

Ensure you are following cold email best practices

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

Essentially, you want to always personalize your email, and approach your prospect with a well-researched, value-focused offer. Have a clear call to action and 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 make sure they’re offering the best possible value to their prospects 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, Warm is here for you.

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