Optimize Your Spam Filter Resistance – Quick Guide


According to research by MarketingSherpa, one out of every six emails never reaches the intended recipient, because it is blocked mistakenly by a spam filter. And according to Visualize.Yahoo, the spam filters in Yahoo! Mail put a halt to four out of five emails during their email verification process.

At GetResponse we strongly believe that the only way to ensure that your emails get delivered is to use permission-based email marketing. It is the key to maintaining an impeccable reputation and the highest possible delivery rates.

However, in their war against spammers, ISPs continue to develop new verification tools that often cause your legitimate campaigns to be junked by spam filters accidentally, even if your emailing policies comply with all the legal requirements and industry best practices.

The confirmed opt-in is no longer just optional; it’s now the foundation for sending legit emails and maintaining your reputation with recipients and major ISPs.

What’s next?

Visualize.Yahoo in a spectacular way illustrates their email verification system with checkpoints a message needs to pass in order to reach inboxes. Those are:


  • IP address
  • links included
  • sender’s reputation
  • email content
  • recipients’ reactions



Any message that fails to pass even one of the verification steps is flagged instantly as spam, which, as they claim, gives them a 4-to-1 ratio of spam to valid email. With approximately 5.6 billion emails processed every day, this represents an enormous load of emails that are rejected by spam filters due to various reasons. So if you don’t pay enough attention to your anti-spam policies, your campaign might be among those 4 billion rejected emails.

Yahoo’s verification criteria are much like those of all the major ISPs (although Gmail keeps theirs secret, while those from SpamAssasin are publicly available on their website).

Let’s see what they are.

Spam Words

Visualize.Yahoo’s most thought-provoking feature is the animated stream-graph of trending keywords vs. spam words.



However, the fact that many (free, sale, today, last, day, etc.) are both “good” and “bad”, indicates that spam tests reach far beyond the spam words checks. Email verification is a far more complex procedure that all email elements are subjected to. Check out which words and phrases can get you in trouble.

Sender’s Reputation

Your reputation with ISPs and with your recipients is the most powerful factor in guaranteeing seamless inbox delivery. Therefore:

  • Use a valid sender’s domain;
  • Use a consistent, recognizable sender’s email address;
  • Ask your subscribers to whitelist you;
  • Don’t send emails to stale addresses;
  • Avoid batch-and-blast emails – break lists into segments;

IP address

Every internet protocol (IP) address has a reputation of its own. And this reputation can be impacted by factors such as:

  • irregular sending patterns
  • outgoing volume
  • poor statistics
  • shared vs. dedicated IP space.

Since building IP reputation involves a lot of advanced solutions, such as feedback loops and email authentication, it’s best to use a reliable email service provider (ESP) that can safeguard the good reputation of your IP.


Email verification refers not only to subject line and spam words checks. Nowadays the following elements of your email are also routinely tested:

  • HTML coding
  • image-to-text ratio
  • word collocations, position in text, and frequency of use
  • Internet domains mentioned
  • links included
  • disclaimers
  • ads included
  • “cute” spelling (e.g. “S.P.A.C.E”)

…and many others.


User-Defined Filters

These probably are the most difficult hurdles to pass, because they are based on the actions and engagement of recipients and user community-based reporting to block unwanted email. They actually “learn” what users consider spam and use that knowledge to determine which messages to allow and which to boot. The typical patterns are to:


  • block a message that has been reported as spam by one user for that particular user’s inbox;
  • allow a message only for a particular inbox if its user “un-spams” it;
  • block a sender for everyone if the sender (or a message from the sender) has been reported as spam by many recipients;


The only solution here is to improve your brand reputation and recognition by e.g.:

  • sending whitelisting request
  • branding your “From” field and subject line for better recognit
  • sending highly relevant emails
  • running re-engagement campaigns
  • removing inactive subscribers regularly
  • providing transparent unsubscribe options
  • and sticking to many other best practices procedures.

So, how to make campaigns spam filter resistant?

The easy answer is: never send spam.

However, the definition of spam is being updated constantly, as ISPs come up with new spam-filter algorithms to prevent abusers from invading inboxes. That’s why marketers need to adjust their strategies constantly to ensure compliance with the most recent regulations and technologies.


Before you get help from professionals, (yes, you can outsource such solutions) you should implement the most obvious strategies:


  • use only double-opt in subscription procedures;
  • test your messages in the major ISP inboxes before you send to entire lists;
  • keep up-to-date with the most important laws and regulations, such as CAN-SPAM;
  • build a good reputation for your brand and keep your recipients engaged;


This is just a start. For more tips to seamless inbox delivery, download our free guide: Email Deliverability from A to Z.

And if you’d like to share your experiences and tips on avoiding spam filters, take the floor and post a comment. They’re always welcome.

Email Deliverability From A To Z

Download our free guide and learn how to make sure that your deliverability is pitch-perfect with tips from a deliverability manager at GetResponse. Nurture relationships, learn how to tackle list building, and make sure that the content you send doesn't raise up any flags.