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ISP and Email Deliverability: How To Hit The Inbox Always

As you delve into email deliverability and avoid poor emails that bounce, ISP is an acronym that will undoubtedly come up.

An ISP or internet service provider like T-Mobile or AT&T determines whether your emails are worthy of reaching its customers’ inboxes, so you must get on an ISP’s nice side and stay there.

This guide will unveil the role of ISP in email deliverability, including what ISPs do, how they’re different from ESPs, why they might block your emails (and what to do about it), and how to build your reputation.

What Is an ISP, Anyway?

Let’s talk further about internet service providers.

ISPs are companies or organizations that provide the internet. They can be privately or community-owned or nonprofit, although the former is more common than the latter. The services an ISP renders are colocation, Usenet, web hosting, domain name registration, internet transit, and secure internet access.

The biggest ISPs in America are Verizon, AT&T, Charter Communications, and Comcast. Other major contenders are Google Fiber, Cox, CenturyLink, and Time Warner Cable. If you have the internet, you have an ISP.

You can find your ISP by opening your preferred internet browser and using the IP Lookup Tool.

ISP Mail Service Infrastructure – What You Need to Know

ISPs utilize an infrastructure or architecture comprised of several parts. Let’s review.


Internet users can’t go online without a router, which uploads and retrieves data. Routing keeps data in its lane, so to speak, ensuring that only the intended parties have access to it.

Domain Addresses

A domain address or name is an internet identifier. Each address is unique and connected to an IP address.

Looking up domain names makes it easy to find specific destinations on the internet rather than remembering IP addresses, which anyone would find challenging!

IP Addresses

An IP address, such as an IPv4 address, helps route users to specific points online.

Points of Presence

Points of presence (PoPs) across network access points allow you to use your internet service provider to go on the internet. PoPs can control network bottlenecks and reduce latency.

Fiber Optics

A fiber optic cable network sends data at lightning speed across the internet. ISPs rely on fiber optics to manage network traffic.

Network Access Points

Network access points, or NAPS, allow ISPs to share data across and between networks.

Read also: How To Avoid Spam Filters For Better Email Deliverability

ISP vs ESP – What’s the Difference?

Now that you understand ISPs, let’s review how they’re different from ESPs.

An ESP is short for an email service provider. These services are email marketing software that send emails to broad groups or specific segments.

ESPs will often provide additional services beyond sending emails that make them worth the monthly subscription cost, including audience segmentation, A/B testing, email analytics, email templates, personalization, AI, and email automation.

Some popular examples of ESPs are Brevo (formerly known as Sendinblue), ConvertKit, Campaign Monitor, ActiveCampaign, Constant Contact, Mailchimp, and HubSpot. If you want an affordable alternative to these, you must try EngageBay.

You can use an ESP to send an email to your audience, but the ISP will determine if the email reaches its destination.

Read also: Avoiding The Spam Folder: An Intro To Email Deliverability

7 ISP Factors That Affect Email Deliverability

When you click “send” on an email (or the automation workflow in your ESP does it for you), where does the message go? The ISP will check the message, filtering it for spam and other dangerous content.

The email goes no further if it gets stuck in the spam filter. The recipient might see the message if they check their spam emails (such as to delete them and clear their inbox), but they’re unlikely to open it, as spam messages are rarely trustworthy.

Should the email not get stuck in spam by an ISP, it will reach the recipient.

So, what kinds of factors do ISPs look at to determine which emails go through and which get blocked? Let’s review.

Sharing IP Addresses

Your IP address is a unique stamp. More specifically, it’s a sending domain that you use for emailing others.

If you have a designated IP address, all your business emails will come from that IP. It doesn’t matter whether you send 100 emails per day or 10,000 (if your ESP lets you, that is); they use one IP.

However, some businesses opt for a shared IP address. As the name implies, this IP doesn’t belong to you alone. Other businesses or individuals share the same IP.

Having a designated IP means you’re in complete control of your sender reputation, as you’re the only one engaging in activities on that IP server. Sharing an IP means accepting the consequences of whatever those on the server do.

In short, if they send spam emails or engage in other bad email practices, your sender reputation suffers for it. You don’t even have to personally engage in these email faux pas. Being attached to the shared IP is enough to tank your reputation.

Of course, shared IPs aren’t always troublesome. Carefully selecting your IP partners ensures you can maintain and even increase your sender reputation on a shared IP server.

A shared IP is sometimes beneficial for startups because they have a small enough email volume but no sender reputation. Sharing an IP will increase the sender reputation faster than using a designated IP.

Read also: Bounce Rates and Email Deliverability – A Simple Guide

Emailing non-permission-based lists (cold email in bulk)

Check your list permissions before launching your next email campaign. ISPs will determine your email deliverability based on the permissions of your email lists.

For example, a non-permission-based email list is a list of email contacts of people who haven’t opted in, or at least not confirmed their subscription. Since this comes as cold email, when done in bulk, it can affect your open and click-through rates.

More severely, you’re likely to get your messages marked as spam, and you’ll find many of these users will unsubscribe.

These factors will damage more than your email marketing campaign but your sender reputation. Your email deliverability will go down the tubes.

Read also: Unlocking The Secrets Of Sender Reputation: Your Key To Email Deliverability

Getting blacklisted by email recipients

Blacklisting is as serious as it sounds. A sender domain earns a spot on the blacklist via an ISP if they send spam.

Once you’re blacklisted, your future emails will go in the junk or spam folders even if you send legit content.

How do you get blacklisted? Your IP data gets placed on a DNS blacklist, and ISPs stop your emails whenever you send them.

Read also: Understanding Mailer Daemon: How to Manage Bounce-Back Emails

Emailing spam traps

We discussed spam traps in our intro to email deliverability, so here’s a recap.

A spam trap is typically an abandoned email address. These addresses often go defunct innocently.

For example, if george.h@companyxyz.com leaves Company XYZ, George won’t check his email there anymore. He probably doesn’t even have access to it.

However, it’s not like George told you personally that he’s leaving, so you can continue emailing him at his Company XYZ email address even though he’s now using georgeh@companyabc.com. It’s crucial to verify email addresses regularly to avoid such issues.

Spam traps aren’t always so innocuous, though. Sometimes, a spam trap is a new email address made for the specific purpose of bringing in spammers.

Regardless of why you have spam traps on your email list, emailing these addresses will damage your sender reputation, causing ISPs to block your emails.

Read also: IP Warming And Email Deliverability: Here’s What To Know

Sending low-quality content

Sender reputation is strongly based on the content you send, so it’s worth paying attention to what you send out to your various audience segments.

So, what does low-quality content entail? It’s irrelevant to your audience. Maybe you send mass emails to your entire broad subscriber base, or perhaps you segment your audience but don’t personalize the content they receive.

Low-quality emails lack professionalism. The links look fishy and can take the user anywhere if they click them. The emails don’t engage and likely aren’t automated. They include JavaScript, embed tags, and other email code red flags.

You might leave your tags open and use link shorteners too. These kinds of behaviors will prevent your subscribers from seeing future correspondence from you, as your ISP will send your messages straight to the spam filter.

Read also: Email Sunsetting Policy 101 For Beginners

Having a high bounce rate

Email bounce rate refers to how many of your messages don’t get sent to the recipient.

You’ll know your emails bounce because you’ll receive an email system error message. This automated message will tell you the email wasn’t delivered.

A bounce rate of 50 percent and up is high, and the greater yours is over that, the worse off your email marketing campaigns are.

Read also: The Impact of Blacklists, Greylists, and Whitelists on Your Email Deliverability

Excessive email volume

ISPs institute an email-sending limit. If you surpass that limit, your messages automatically bounce, even if you’re emailing legitimate, engaged, subscribed email users. This is a soft bounce but a bounce, nonetheless.

Your email send limit resets daily to weekly and is based on your average sending volume.

Read also: How to Set Up DMARC: Safeguard Your Business from Phishing

Low engagement

ISPs also check for email engagement to determine whether an email should reach a recipient’s inbox. Engagement refers to metrics like opens, spam complaints, unsubscribes, and clicks.

Some of these metrics, like opens and clicks, are positive attributes or signals to an ISP. Unsubscribes and spam complaints are negative attributes.

If your negative attributes substantially outweigh your positive attributes, your ISP could stop your messages from going through.

Read also: Email Deliverability: 7 Tips To Get More Clicks, Sells & Signups

ISP Tips to Increase Email Deliverability

You’ve monitored your latest email campaign only to realize with horror that your messages aren’t going through. After a process of elimination, you determined that your ISP must be blocking your emails.

This negates the money you spend on an ESP and prevents you from engaging with your audience. Therefore, you must negate any ISP issues before you send another email.

Here are some suggestions to help you do just that.

Have a routine to clean up your email list

The first task on your list is purging your email list.

You can use an ESP to do this or manually complete the task. Earmark any email addresses that have bounced recently and remove them from your list.

Reach out to audience members who don’t have all their permissions set, sending an explanatory email about how to fully opt-in. However, if nothing changes with these subscribers, you should strongly reconsider sending them further correspondence.

Read also: Top Tools & Strategies To Improve Email Deliverability Rates

Create cool re-engagement campaigns

Now is also a great time for a re-engagement email campaign. Reengaging with the subscribers you haven’t contacted in a while helps you determine who’s willing to engage with your content and who isn’t.

After your re-engagement campaign draws to a close, use the data you’ve gathered to prune your email list again.

Read also: Learn To Manage Email Bounce Codes In 10 Minutes

Don’t purchase email lists

Image courtesy of Giphy

Purchasing contact lists is tempting for startups and small businesses with a teeny-tiny customer base. You can have hundreds to thousands of subscribers instantly by spending some money upfront.

However, buying your contacts creates all sorts of problems. You don’t necessarily know if the new subscribers will enjoy your products or services. If they don’t, they’ll unsubscribe, leaving you without an email list.

You also have no idea who is on your email list, how active they are, and whether the email addresses are spam traps. Gambling like this will almost surely harm your sender reputation.

You’re much better off acquiring email subscribers through legitimate means, even if it takes longer.

Read also: Email Seed Testing: A Time-Tested Email Deliverability Hack

Make it effortless to unsubscribe

Don’t let your sender reputation fall by the wayside in the quest to grow your contact list. Give your subscribers an easy option to unsubscribe.

This might initially seem counterintuitive, as you will just shrink your subscriber list rather than grow it. Perhaps so, but you’ll preserve your sender reputation.

After all, if your sender reputation is bad enough, it doesn’t matter whether you have 1,000 email contacts or 10,000. You can’t reach any of them because your ISP will block your emails.

You don’t want people subscribed who won’t engage or aren’t reading your emails. Your ISP will not view these actions positively, and it will drag down your sender reputation.

Read also: Email Feedback Loops: How They Work, How to Set Them Up, and Some Tips

Refine your email content for quality

This is an excellent time to revise your email content and improve its quality. Begin by reviewing the code. Don’t use JavaScript, and triple-check that you don’t have open tags or other coding errors.

Next, revise the look of your messages. Switch up the generic email templates for customized ones. This also benefits your brand and increases your trustworthiness, as your messages have a cohesiveness that’s also seen on your social media accounts and website.

Segment your audience and produce personalized content for them.

Double-check that the messages slated to reach each segment have the right messaging and product/service recommendations before you email them.

Read also: The Impact Of Spam Complaint Rates On Email Deliverability (And How To Fix It)

Increase email volume gradually, not suddenly

Do you plan to start a large email campaign but worry about exceeding your ISP-set email-sending limit? That send limit is flexible in that it increases based on your email volume, but within reason.

The best course of action is to take it slow in ramping up the number of emails you send out. For example, if you have a holiday campaign planned to begin in December, start increasing your email send rate weekly throughout November.

This way, by the time December rolls around, you can send the required number of emails without getting soft-blocked by your ISP.

Read also: Bounce Back Emails: What Are They And How To Fix Them

Bottom Line

An internet service provider or ISP is the ultimate authority on whether your emails get sent. The key to improving email deliverability is to think like an ISP. Eliminate spam traps from your contact list. Stop sending irrelevant, mass emails.

Segment your contacts and send only personalized emails.

Make your emails professional by revising and cleaning up code and using custom templates. Gradually build up your email sends to avoid soft bounces, as a high bounce rate indicates a poor sender reputation.

These small measures will pay off when your recipients receive your messages with no problem!

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