In this guide, we’ll show how you can maximize the return on your email marketing investment, by ensuring your campaign content, subscriber lists and account settings are geared towards getting past Internet Service Provider (ISP) spam filters and ultimately, into inboxes.

By following a few practical tips, it is possible to build and maintain a good reputation with your ESP, various ISPs, and your subscribers.

Chapter 1

Getting started

There's a common myth that once you team up with an Email Service Provider (ESP) like Campaign Monitor to create and send email campaigns, you can leave all your list management and delivery concerns to them. As much as we all wish this was the case, getting into inboxes is very much a team effort - both senders and the companies that help create and deliver their emails must work together to ensure that as many messages as possible make it to recipients.


ESP - Email Service Provider. A company that helps senders create and deliver email campaigns
ISP - Internet Service Provider. In the context of email, these are the companies that own the domains and/or mailboxes that emails get sent to, eg. Gmail,, Comcast

"You must be new around these parts..."

Now, we didn't use the word "reputation" by accident earlier on. Reputation is a common term in email marketing, used to describe the relationship a sender or ESP has with the other parties that handle an email - from the time it's sent, to the time it makes it to the recipient. The way this relationship affects your campaigns isn't entirely different from say, how the relationship you have with your work colleagues influences how they treat your day-to-day email. For example, if you have a reputation for sending concise and informative messages, then the folks who receive your email are likely to give them more attention and care than if you have a knack for forwarding joke emails all day.

Likewise, if the emails you send include quality content and regularly get responded to (other than being marked as spam), then it's likely that you'll gain a good reputation with both your ESP and with ISPs like Gmail. All this contributes towards optimal delivery rates for your campaigns.

That said, things can still go wrong, even if you have a good track record. Sometimes, an ISP or spam filter en route will unexpectedly block or bounce incoming email for one of many reasons. But at least you can minimize the chances of this happening and worst comes to worst, be in a good position to get delivery issues resolved promptly with your ESP.

Chapter 2

Improving your email content

Spam filters are similar to benevolent, yet mad dictators - while they mean well, they're complex, seemingly fickle and totally opaque in their thinking. However, over time, we've come to learn a few things about what make them tick - so, in this chapter, we've collected a few practical tips to help you abide by their rules.

Similar to what they say in crime shows, to get around spam filters, you've got to think like a spam filter. So first of all, lets start by looking at what 'spammy' email content looks like.

Avoid using URLs as text, or shorteners

While adding a URL as text to email content may seem like a really straightforward way to denote a link, it tends to set off all sort of spam filter alarm bells. This is because of the tracking links used when a campaign is sent from an ESP often differ from the link text itself; prior to send, it's not uncommon for a URL contained within an <a> tag to be changed so that clicks can be recorded. For example:

<a href=""></a>

May be changed to something like:

<a href=""></a>

While this is common practice, to spam filters, it simply looks like the sender is trying to 'hide' the link. It's better to use any other text for your links - more often than not in this scenario, link text like 'visit our site' will do.

For similar reasons, links generated using URL shorteners like also raise flags. Hiding a URL using a shortener is a common technique used by spammers and so these links are treated with suspicion, either when used as text or within a link.

Balance images with copy

Another scenario in which a message may look spammy is when an HTML email simply contains one big image, or has a lot of images and very little copy. While not so common these days, 'all-image' emails have been previously used to display messages that would otherwise get filtered, so it's wise to avoid sending emails like this.

The other advantage to minimizing the use of images is that you can avoid having your message not display when images are blocked in the inbox. Previous research has shown that less than 50% of email recipients use email clients that display images by default (ie. without a prompt), so balancing text and images makes a lot of sense.

Use a valid reply-to email address

Using an actual, existing mailbox to collect replies to your newsletters isn't just good manners - some ISPs like Gmail actually look into recipient behavior after an email lands in the inbox. When senders use reply-to email address like "", they are discouraging the kind of interaction that indicates to spam filters that the email message is valuable. In essence, encouraging recipients to open, click and reply to campaigns will improve the chances of both current and future campaigns making it through.

Be consistent with 'From' details

While it may seem like a minor detail, using the same 'From' name and email address on your campaigns is an notable part of getting your campaigns successfully delivered, according to Gmail's sender guidelines. Whether this is because Gmail uses this to calculate some kind of reputation, or because it helps recipients identify campaigns is unknown, but both are good reasons to stay consistent.

Chapter 3

Maintaining healthy subscriber lists

There are a number of reasons why emails may bounce following a send. While there's little that can be done about bounces that are a result of folks abandoning their accounts, quitting jobs or configuration issues, bounces as a result of invalid email addresses can be minimized, or avoided altogether.

Another factor in maintaining a healthy subscriber list is ensuring that spam complaints are kept to a minimum. In this section, we'll look at some of the techniques used to tackle both these issues.

Confirmed opt-in for the win

To many senders, the two approaches to adding new subscribers to a list, being single and confirmed (or double) opt-in, are a bit of a mystery. This is largely because they seem like relatively advanced concepts, when in fact, the differences are pretty straightforward. For example, let's say you have a form on your site that's pointing to a subscriber list. If it's single opt-in, it will accept new subscribers immediately after the form is submitted. However, confirmed opt-in lists include an interim step, which usually involves an email being sent to the subscriber's email address first. Once a link in the email is clicked (thus confirming that the email address is valid), the subscriber is added.

There are advantages and disadvantages to each approach. While single opt-in is simpler than confirmed for both email senders and subscribers, it does leave lists wide open to collecting invalid email addresses - either as a result of honest mistakes, or automated nasties like spambots. On one end of the scale, these invalid email addresses can be a mild annoyance, but on the other, they can be an expensive problem, impacting your campaign metrics, delivery rates and bottom line.

For this reason in particular, there's a lot of value in using a confirmed opt-in list. Not only does it ensure that new subscriber details are valid, but in the process, it's great for boosting your delivery and engagement rates, too. If getting into the inbox is a priority, then confirmed is the way to go.

Stay in touch... regularly

Second to sending too regularly, not emailing enough can be a source of spam complaints. Subscribers can forget that they've subscribed to a given list, which is why we generally recommend that you don't email someone if they haven't been contacted in the last 2 years.

If a subscriber list doesn't get any use for roughly 6 months or more, we usually suggest sending a periodic reminder or re-engagement email. These emails can also provide the option of reconfirming the subscription, to ensure that everyone on the list really is keen to keep receiving emails from the sender. Here's a great example of a re-engagement campaign from Panic:

Overall, your best bet is to have a plan to send relevant, targeted content on regular intervals, so your lists don't go stale.

Don't buy lists!

Now, this one may seem pretty cut-and-paste to many, but it can't be denied that the idea of suddenly having thousands of new people to email is appealing. So lets get this straight - buying subscriber lists is definitely a no-no, regardless of how 'targeted' or 'interested' a vendor claims their lists may be. In many countries, sending without gaining explicit permission from all subscribers is against the law. Even if someone does get to the point of emailing a purchased list, they'll be sure to receive not only abysmal response rates (think how many times a sellable list has been used and abused!), but likely poor delivery rates, a slew of spam complaints and strife from their ESP's compliance or abuse team as well. Now, that's what I call bad value for money.

Finding out whether or not you have permission to email your subscribers can be a grey area at times, so we've produced a guide to Understanding Permission to help with this task. When in doubt, your best bet is to not email the list at all.

Be proactive about whitelisting and engagement

As we touched on earlier, getting your subscribers to do something (open your email, click, reply or forward) when they receive your campaigns is a great way to ensure future emails make it into the inbox. At a time when Gmail and others are very clever at analyzing how subscribers interact with their email, you can count on their spam filters to make assessments on whether or not an email should be delivered, based on prior history. Compelling copy, relevant content and offers can play a big part in generating subscriber engagement with an email campaign, as can making it clear that all replies will be read and responded to by real human beings.

Another approach that is commonly used to circumvent spam filters is to request that recipients add the sender's email address to their email client's 'Safe senders list' or 'Whitelist'. Adding a sentence to your email like, "To ensure you receive future emails, please add me to your address book" and linking to a whitelist instructions page can be very helpful.

A common cause of delivery issues when sending to your own domain (ie. From, to is that spam filters can interpret these campaigns as phishing attempts. The remedy is to whitelist your ESPs mail servers - here's more on why this happens and what can be done.

Chapter 4

The real deal: getting authenticated

Now we've looked into optimizing both our email content and lists, the final bit of advice is to look into something a little more technical, being authentication. In short, authentication is a way of showing that an email really has been sent from the person it claims to have come from. There are currently 3 standards accepted by the major ISPs, being:

  • SenderID
  • DKIM
  • SPF

What's important to know is that different ISPs use one, or a combination of these standards to check legitimacy of the sending server (and therefore, the sender). While each ESP has their own approaches to authentication, Campaign Monitor have focused on making it as easy as possible for you to work with these standards.

Create a consistent cadence for your #emailmarketing. Irregular email increases spam complaints

While this has been by no means an exhaustive guide to getting your email campaigns into subscriber inboxes, these practical steps will be sure to boost your chances of making it past spam filters, while avoiding list cleanliness and delivery issues. For a bit of further reading, we really recommend giving Gmail's sender guide a read, as well as our guide to Understanding Permission. With this knowledge, you'll be sending with the confidence that you're giving your campaigns the best possible chance of landing in the inbox.

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