7 Ways to Prevent Your Emails Being Blocked as Spam

949308_61702022Earlier this week I was talking with a client and discovered that they had not received an important email that I sent them that morning.  After a brief investigation we found the email in their Outlook Junk email folder.  This was unusual because, of course, we send emails to each other quite frequently.

I checked it out and was able to determine why that particular email got marked as Junk email, made a configuration change, and emails have been delivering fine ever since.  The experience left me thinking about some of the ways that perfectly legitimate email might be marked as spam, and the steps that can be taken to avoid those situations.

Flagged as Spam by End User Email Client

An email recipient can flag an email as spam in a number of ways.  If their email client has built-in junk filters then they can simply mark it as junk email in their client and your emails will not reach their inbox.

For businesses with Exchange Server 2007 and Outlook on the desktop this goes a step further due to a feature called Safelist Aggregation.  This feature aggregates each individual user’s personal safelist and blocklist information onto the server itself, so that an email address marked as spam by one user is also blocked for other users.

Unfortunately this can happen when people either forget that they signed up for a particular newsletter or promotion, or they simply decide that they no longer want to receive it, and instead of unsubscribing they mark it as spam.

You can avoid this situation by always making it clear how you acquired a recipient’s email address and how they can go about unsubscribing from your emails.

Submitted as Spam to Email Security Vendor

Most vendors of anti-spam software will accept spam submissions from customers to help them continually update their signature databases for the latest spam emails.  As with the previous point it is not uncommon for people to submit legitimate emails as spam simply because they forgot that they signed up to receive them.

The solution to this is the same, as a vendor is unlikely to update their product to begin treating your emails as spam if you are making it clear that the recipient signed themselves up and can unsubscribe at any time.

Sending from Dynamic IP Address

Because of the problem of spam botnets on the internet many email servers are configured to use block lists that will reject email from ISP customer IP addresses, which are usually dynamically allocated ranges of IP addresses.  The problem here can be twofold – either the IP you have been allocated was previously known to send out some spam, or the IP is within a dynamic IP range that is on a block list, or sometimes both.

The solution to this can be to either acquire static IP addresses from your ISP to run your email server, or to use your ISP as a smart host to relay outbound email from your server to the internet.  Most ISPs offer this service to customers.  Because the smart host IP address is more trusted and less likely to be on a blocklist your emails are more likely to be delivered.

Sending from an Open Relay

Similar to the previous point if your server has been misconfigured or compromised by hackers and is known to be an open relay then you will find your emails getting blocked by anyone using one of the major blocklist providers.

Failing SMTP RFC Requirements

Sometimes email will be treated as spam simply because your server does not meet all of the requirements specified in the SMTP RFC.  I know of at least one major US ISP that outright blocks your IP address if it does not have a correct reverse DNS entry, and getting unblocked means jumping through quite a few hoops.

Failing Anti-spam Framework Requirements

There are several anti-spam frameworks available to email administrators, and although none have become an official standard they are often used by email admins as one of many factors in assessing emails for spam.

Most framework requirements are very simple to implement, and complying with them only improves your chances of successful email delivery so it is worth the time to configure your servers for them.

Sending Spam

This last one might seem obvious but if you have actually sent some spam in the past then the likelihood of future emails being treated as spam is much greater.  Where many companies fall into this trap is when they engage in email marketing and don’t pay attention to the legal and ethical requirements associated with this type of activity.

First and foremost make sure that you are conducting your email campaigns in compliance with any spam legislation in the regions in which you operate.  Secondly remember that spam is often in the eye of the beholder.  You may have legally acquired a person’s email address and sent them marketing emails but if your content is “spammy” to them in any way then it will be treated as such.

As you can see spam is a complex issue not only from the preventative side but also from the deliverability side.  If you spend the time to consider these elements and implement the correct solutions on your servers and within your business you should find yourself able to communicate with your customers via email without fear of messages being blocked.

Written by Paul Cunningham

Paul lives in Brisbane, Australia and works as a technical consultant for a national IT services provider, specialising in Microsoft Exchange Server and related messaging systems.


  1. AvalonNYC · July 31, 2010

    I would check your information about “safelist aggregation”. I believe it is incorrect that “an email address marked as spam by one user is also blocked for other users”. “Aggregation” is not performed from users to enterprise. The only aggregation is that the user’s lists are aggregated (actually copied) from the mailbox to Active Directory, where they are available to the transport agents and so can be applied earlier in the transport chain, thereby decreasing false positives and saving work. The user’s safelists (and blocklists) are still applied only to that user.

  2. daniel yeboah · July 19, 2012

    my emails are received by my recipient as spam

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