Email providers and Internet Service Providers (ISPs) check all incoming mail to decide whether they'll consider it valid email worth delivering to their subscribers, or whether it's spam email to be blocked, bounced, or delivered to a spam or junk mail folder rather than the recipient's inbox.

Spam filters and and spam detection methods are constantly changing. The rules by which email providers and ISPs rated email as spam just a few years ago are no longer all that senders have to be aware of. Subject lines and the actual content of email are still reviewed by spam filters, but that's just the beginning of the process.

There are a number of additional factors and criteria that enter into spam filtering. We'll go over a number of them here and explain how you can manage your audience and subscriber lists to try to avoid your emails being rated as spam.

Email Source and Sender Reputation

Email providers and ISPs look at the IP addresses that email comes from - who is sending email, through what providers or companies? IP addresses or email systems may be identified or recognized as having good or bad reputations, and email carried on those IP addresses or systems can benefit or suffer from that reputation. Questline uses dedicated IP addresses and domains to make sure that email for our clients is separated from email sent by other companies. Additionally, we use domain authentication methods to improve deliverability and reputation.

Additionally, the sender identified for an email by the from/reply to email addresses has a reputation. Multiple senders can share IP addresses or servers, but have their own reputations based on the email they send. 

Spam Complaints

Emails that successfully reach subscribers can still be marked as spam by their readers. Many modern email providers include a button on the toolbar that allows subscribers to report a message as spam. Making sure that you're sending to subscribers who want to receive your email will help limit these spam complaints. 

Emails that receive high amounts of spam complaints from readers can be flagged across shared reputation networks. An email provider or ISP can decide to block emails still being processed if their subscribers make sufficient spam complaints early in the delivery of that campaign, and they can even share this spam rating across reputation networks. That means that spam complaints at one email system can affect other email systems as well.

Spam Trap Hits

Spam Traps are email addresses that have either been abandoned or specifically setup to catch email senders with poor list practices. These email address no longer belong to real readers (or never did in the first place) and are used by mailbox providers to trap senders with poor list hygiene practices. For a detailed explanation, you can read more here. Spam Trap addresses are often added to subscriber lists if they're obtained 'non-organically' - purchased or appended lists can include fake addresses meant to 'trap' mailers who use these methods. Some spam traps can simply be typos or email addresses abandoned by users in the past. These addresses don't 'bounce' as invalid addresses, but the intended subscriber will never actually read the email you send them. To get rid of spam traps in your lists, we advise regularly cleaning your lists of inactive readers.

Reader Engagement

More and more email providers and ISPs measure the engagement of your audience and use that as an indicator of whether your mail should be delivered in the future. Engagement measures how many people are not just receiving your email, but actually opening it, reading it, forwarding it, or clicking links. If you send to an audience that is largely inactive and letting your email go unread email providers/ISPs will notice. And after some time sending to an unengaged audience, they can decide that your audience isn't really interesting in reading what you're sending them. This can result in future emails from you being marked as spam, and sent to spam folders or blocked entirely. When this happens, even potentially engaged readers might no longer see your emails.

Just like spam traps, this can be helped by regularly cleaning your subscriber lists of inactive readers. We can work with you to determine which subscribers have received your emails and let them go unread. We can weigh several factors, including target audience and sending frequency, to suggest lists of subscribers for suppression from your email campaigns.


The volume of email you send out, and the frequency at which you send to an audience, can contribute to spam profiling. Keeping email sent out regularly and spreading out excessively large sends can help you look like a stable and regular email sender. 

Content Analysis and Fingerprinting

As explained above, email spam filtering these days is about more than subject lines, fonts, images, and email body text, but those do still enter into the equation. Emails are often scanned by automated systems and assigned a 'fingerprint' that allows the email and similar emails to be tracked by reputation networks shared across email providers and ISPs.

The content of emails does contribute to its spam profile and fingerprint, but additional factors, such as reader engagement and spam complaints, often weigh more heavily on the spam score of an email today.