The Complete Guide to Mastering Email Deliverability

Intro

You’ve carefully designed your email, typed a perfect copy that speaks directly to your audience, and finally hit send. Yet, despite your best efforts, it may not be making the impact you hoped for. That’s probably because it’s not even getting seen.

How, you ask? Your emails may be landing in the spam folder. Frustrating, isn’t it?

In this guide, we’ll explain what email deliverability means, explore the factors that affect it, and share practical tips to help your emails reach the inbox every time.

What is email deliverability?

Email deliverability is the ability of your emails to successfully land in your recipients’ inboxes, rather than being filtered into spam, junk, or promotions folders. The right email marketing software can significantly improve deliverability by helping you follow best practices and monitor inbox placement.

Email deliverability vs email delivery

One term that often gets mixed up with email deliverability is email delivery. They might sound the same, but they’re very different.

Email delivery refers to the successful transmission of an email to the recipient’s mail server. This depends on technical factors like having valid email addresses, correct domain settings, and proper authentication protocols in place. If the server accepts the message, it is considered delivered, even if it never reaches the recipient’s main inbox.

Email deliverability goes a step further. It measures whether your email actually lands in the recipient’s primary inbox, instead of getting filtered into spam or the promotions tab. Inbox placement is influenced by factors like sender reputation, engagement rates, and the quality of your content.

5 things that affect email deliverability

Inbox providers (ISPs) are the ones who decide where your emails go. If they trust your email, it goes to the inbox. If not, it could land in spam or not get delivered at all.

They make that decision based on a few key factors:

1. Sender reputation

Sender reputation is a score that email service providers (like Gmail, Outlook, Yahoo, etc.) assign to you based on how trustworthy and reliable your email-sending behavior is.

Some of the key things that influence your sender reputation include:

• How often do people open or interact with your emails
• How frequently your emails bounce or get flagged as spam
• Whether you're following proper sending protocols
• How clean and valid your email list is
• If your emails are hitting spam traps or blacklists

The better your sender reputation, the more likely your emails will reach the inbox. But if your score dips below a certain threshold, inbox providers might start pushing your emails into spam or block them completely.

2. IP and domain reputation

When you send an email, it comes from an IP address (a unique set of numbers tied to your email server) and a domain (like yourcompany.com). Both of these are monitored by inbox providers to decide whether your message should land in someone’s inbox, go to spam, or get blocked.

In terms of IP addresses, inbox providers track how that IP has been used in the past. If the IP has a history of sending spammy or irrelevant emails, it gets a bad reputation. If it consistently sends clean, well-received emails, its reputation stays strong.

Domain reputation works similarly, but it follows your domain, not just the IP you use. This means even if you switch platforms or change IPs, a damaged domain reputation will remain damaged and lead your emails to the spam folder.

3. Email authentication

One of the first things a receiving server checks is whether your email is authenticated. Without proper authentication in place, your emails may appear suspicious and be filtered out, even if your content is clean.

There are three key authentication protocols:

• SPF (Sender Policy Framework): This lets you define which servers are allowed to send emails on behalf of your domain. If an unauthorized server tries to send from your domain, it’ll get flagged.
• DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to your emails so inbox providers can verify that the message hasn't been tampered with during delivery.
• DMARC (Domain-based Message Authentication, Reporting & Conformance): DMARC builds on SPF and DKIM. It tells inbox providers what to do if an email fails authentication (e.g., reject, quarantine, or let it through).

These work together to confirm your identity and prevent spoofing. Setting them up helps build trust and protects your deliverability.

4. Spam complaints

Spam complaints are one of the strongest negative signals to inbox providers. When someone clicks “Report Spam” on your email, it tells their email service, “I don’t want messages like this.” And if enough people mark you as spam, inbox filters take notice.

The more complaints you get, the harder it becomes to land in the inbox. There are several reasons why someone might report your emails as spam, including:

• You're sending too many emails too often
• The content doesn’t match what they signed up for
• They forgot they subscribed
• The unsubscribe link is hard to find

5. The content of the email

Your email’s content, including the subject line, preheader, and body, plays a subtle but important role in inbox placement.

Some common red flags include:

• Use of link shorteners like bit.ly
• Non-secure links (http instead of https)
• Image-only emails with no text
• Excessive use of ALL CAPS or special characters in subject lines

Even small tweaks to your subject line or formatting can make the difference between landing in the inbox or getting flagged.

How to test email deliverability

Here are some simple ways email marketers can follow to monitor deliverability successfully:

1. Set up deliverability tools

To effectively monitor your email deliverability, start by using specialized deliverability tools that help you track your sender reputation, inbox placement, and potential delivery issues.

Some common examples of these tools include MX Toolbox and MessageBird’s Inbox Tracker, which provide detailed insights into areas like IP reputation, blacklist status, spam complaints, and overall deliverability trends.

You can also use free online tools like Mailmodo’s DKIM, SPF, and DMARC checker to verify that your authentication records are properly configured and functioning.

2. Run seed list testing

Seed list testing means sending emails to a controlled list of verified, whitelisted addresses you manage. This lets you see exactly where your messages are landing, whether it's the primary inbox, spam, or promotions tab.

The seed list should be similar in scale to your real mailing list to ensure you get reliable insights across different clients and devices.

3. Conduct content filter testing

Content filter testing uses tools like SpamAssassin or Cloudmark to scan your emails for spam-like elements in the subject line, body text, and links. Emails that don’t pass these checks may be flagged as spam or blocked altogether.

Common email deliverability mistakes to avoid

1. Buying email lists

When you purchase an email list, you’re sending messages to people who haven’t explicitly signed up to hear from you. This leads to many recipients unsubscribing from the list or marking your emails as spam.

To prevent this, build your list organically using opt-in methods. Confirm subscribers’ interest through double opt-in to ensure valid, engaged recipients

2. Sending repetitive or similar content

If you keep sending emails with nearly identical or repetitive content, your subscribers are more likely to get annoyed and unsubscribe.

To address this, create unique content for every email campaign. Tailor subject lines and calls to action to maintain reader interest and avoid triggering spam filters.

3. Sending attachments in bulk emails

Attachments are generally meant for one-to-one emails, not mass mailings. Including attachments in bulk emails may not always get you blocklisted, but it increases the chances of being flagged by spam filters.

To avoid this, it’s better to include links to files hosted online instead of sending attachments directly. This keeps your emails lighter and less likely to trigger spam defenses.

4. Using Blind Carbon Copy (BCC) for mass emails

BCC is sometimes used to hide recipient addresses when sending to multiple people. However, many email providers view BCC mass emails as suspicious because they can resemble spam tactics, leading to deliverability problems.

Instead of BCC, send personalized emails using an email marketing platform or ESP. These tools handle bulk sending properly and personalize messages, improving deliverability and engagement.

5. Overuse of images in emails

Emails with too many images and little text can be seen as spammy by filters, which often leads to lower inbox placement rates.

Maintain a good balance and make sure important information is conveyed through text, supported by visuals.

6. Absence of an unsubscribe option

Not providing a clear way to unsubscribe can frustrate recipients, leading to spam complaints and damaging your sender reputation.

To maintain a healthy list, include a clear and easily accessible unsubscribe link in every email.

Final thoughts

Email deliverability is an ongoing work where the more your subscribers engage with your emails, the stronger your inbox placement becomes. That’s why it's important to keep a close eye on your email engagement metrics, including opens, clicks, and bounces, to ensure you’re delivering the right message to the right readers.

Equally important is selecting the right email marketing platform. Platforms like Mailmodo offer features that help automate subscriber management, track email performance, and support re-engagement efforts, making it easier to stay connected with your audience and keep your emails landing where they belong.