Email Deliverability 101 - Guides from the Experts

Do your emails actually reach your intended recipients? Deliverability depends on a mix of technical setup, list quality, content, and sending behavior. This guide, from our Deliverability Services team, covers what actually moves the needle.

Authentication & Sending Fundamentals

Before content or list quality even come into play, ISPs check whether your email is who it says it is.

• Authenticate your domain. Set up SPF, DKIM, and DMARC for every sending domain. These records prove to ISPs that your email is legitimately coming from you and not being spoofed.
• Use one domain per sending stream. Keep marketing, transactional, and any other distinct email streams on separate subdomains. This way, a reputation problem in one stream (say, a bounce spike in a re-engagement campaign) doesn't drag down your other sends.
• Understand what you can and can't see. Senders only see aggregate stats like opens and clicks. ISPs make placement decisions (inbox, Promotions, Clutter, Spam) using signals senders don't have visibility into, including engagement history and spam complaint rates. This is why following best practices matters even when you can't directly verify the effect.
• Design for mobile. A large share of email is read on mobile devices first. Emails that don't render well on mobile lose engagement, and engagement is one of the strongest signals ISPs use for placement.

Build a Quality List

List quality has a bigger effect on deliverability than almost anything else you can control.

• Know your data sources. Not all contacts are equal. Organic double opt-in contacts (people who confirmed via a reCAPTCHA-protected form, then confirmed again by email) are the highest quality. Purchased, rented, or scraped lists are the opposite: they're a leading cause of spam traps and blocklisting, and Act-On does not allow this data.
• Run list hygiene regularly. Email validation services check that addresses are real and catch known spam traps before you send to them. Act-On partners with NeverBounce (real-time validation) and Webbula (historical trap and risk database) for this.
• Sunset unengaged contacts. Recipients who never open or click drag down your reputation and are a common source of recycled spam traps. Remove or suppress them on a schedule rather than mailing to your full list indefinitely.
• Handle unknown lists carefully. If you inherit a list with no history, check its age and past performance, run it through a hygiene service, and test with a small send (under 20% of your daily volume) before mailing it broadly.

Segment by Engagement

Engagement segmentation means grouping contacts by how recently and how often they've interacted with your emails, so you can adjust who gets mailed and how often.

• Define "engaged" based on your cadence. There's no universal window. A sender emailing daily should define engagement over a shorter time period than one emailing monthly. In Act-On, engagement can be segmented on opens or clicks within a defined time period.
• Prefer engaged segments over suppression segments. Building a segment of contacts to actively mail to is more reliable than trying to suppress everyone who's gone quiet.
• Be cautious with re-engagement campaigns. They carry more risk of hitting spam traps than standard sends. If you run one: cleanse the list first, cap it at 10-20% of your daily volume, and sunset contacts after 2-3 attempts with no response.
• Automate sunsetting. Use a recurring list-maintenance program to catch contacts who fall out of your engaged segment and move them to a "do not send" list automatically.

Make a Strong First Impression

Your first email or two sets the tone for the whole relationship, and directly affects whether you end up in the spam folder.

• Reintroduce yourself. Your first email should remind recipients who you are, what content to expect, how often, and where they opted in. Don't assume they remember signing up.
• Send a double opt-in confirmation. Confirming a new contact's identity right after signup improves both deliverability and data quality.
• Follow through on what you promised. If you set an expectation (frequency, topic, etc.), meet it. Recipients are far more likely to unsubscribe than to mark you as spam if you're upfront, but the reverse is true if you aren't.

Write Content That Gets Opened and Clicked

Content affects opens, clicks, and whether ISPs route your email to the inbox at all.

What drives opens

• From address and display name: avoid "noreply@" and other addresses that can't receive replies. Keep the display name recognizable and consistent with your email address.
• Subject lines: clearly state what's in the email. Avoid misleading urgency, ALL CAPS, and excessive emojis or symbols.
• Preview text: treat it as a subtitle to your subject line. Avoid technical artifacts like "Trouble viewing? View online" or raw alt-text bleeding into the preview.

What drives clicks

• Lead with one clear, primary CTA placed above the fold, styled as a distinct button or clearly set-off text (not an image).
• Cut the clutter. Clean layouts with white space consistently outperform cluttered ones with too many competing links.

What can trigger spam filtering

• Alt text on every image, both for accessibility and because ISPs check that alt text matches image content.
• Clean links: use HTTPS, avoid URL shorteners, and make sure visible link text isn't itself a mismatched URL.
• Clean HTML: avoid embedded JavaScript, iframes, mismatched font colors, and sloppy markup in general.
• Avoid classic spam trigger words and phrases like "free," "100%," or "last chance."

Build Your Sending Strategy

How, how often, and how much you send matters as much as what you send.

• Segment by audience type. Know what share of your list is B2B vs. B2C and which ISPs they use (Google in particular has stricter, reputation-driven filtering). This shapes which practices matter most for your list.
• Keep sending volume consistent. ISPs favor predictable, steady sending volume over sporadic spikes. If you're increasing volume, ramp up gradually (a common approach is doubling weekly) rather than jumping straight to your target volume.
• Set cadence by funnel stage. Bottom-of-funnel prospects and onboarding customers can tolerate more frequent email; top-of-funnel and current customers need more space to avoid fatigue.
• Watch for email fatigue. Use fatigue suppression rules and behavior-triggered sends (rather than fixed schedules) to avoid over-mailing any one contact.

Test and Optimize Continuously

Deliverability isn't a one-time setup. What works can shift over time, so testing should be ongoing.

• Use A/B testing to compare subject lines, send times, and other variables. Act-On's built-in A/B testing covers click-throughs; for other engagement metrics, run a split test on a larger list and compare results in Analytics.
• Match what you test to the metric you're trying to move: subject line, from address, and send time affect opens; CTA placement, wording, and layout affect clicks; opt-out link placement and cadence affect spam complaints and opt-outs.

Sending Checklist

If you're interested in working with our consultants to get email support targeted at your unique business and audience, you can get more information from our website.

• Developing Email Strategy to Optimize Deliverability.pdf6 MB
• Testing and Optimizing Your Emails.pdf3 MB
• No Second Chance at a First Impression.pdf10 MB
• Your Guide to Developing a Content Strategy to Maximize Email Deliverability.pdf4 MB
• Act-On_Deliverability 101_Guide to Inbox Placement.pdf1 MB
• Act-On_Deliverability 101_Our Guide to Engagement Segmentation for Improved Deliverability.pdf3 MB
• Act-On_Deliverability 101_Quality Data and Email Hygiene.pdf1 MB