Email marketing is a vital channel for brands to drive engagement and grow their subscriber base. But it only works if your emails actually land in the inbox. That’s where deliverability comes in—it ensures your messages reach the people you’re trying to reach. When email deliverability issues arise, troubleshooting can feel like a mystery. But the good news is that once you identify the root cause, you can take the proper steps to fix it—and start seeing better results. Let’s break down some of the symptoms.
Identifying these symptoms helps narrow down the cause so we can tackle the issue with the right solution.
First, determine the nature of the decline. Are emails landing in spam folders or being rejected outright? Pinpoint when the drop occurred, as timing can offer clues to potential causes.
Have you changed your sending pattern by increasing frequency, expanding your recipient list, or modifying email content? Also, check if the issue is isolated to one ISP or mailbox provider or if it spans multiple domains.
Finally, verify whether your domain or IP has been blocklisted, as this could be a significant factor in declining rates.
If your emails are being marked as spam, verify that SPF, DKIM, and DMARC are correctly set up and aligned. But authentication isn’t the only factor. Assess your sending frequency to ensure it’s aligned with subscribers’ expectations. Sending too often or too little could be part of the problem. Also, sending too frequently to disengaged recipients can have consequences.
To further identify issues and improve deliverability, authenticate your emails and enroll in major ISP monitoring tools like Gmail Postmaster Tools, Microsoft SNDS, and Yahoo Feedback Loop. These can help you spot issues before they impact your deliverability.
If your emails are getting rejected, start by checking the bounce codes—they’ll tell you what’s going on. Temporary failures (4xx) often result from transient issues, full inboxes, or grey-listing – where a receiving server temporarily defers messages to test sender legitimacy. While permanent failures. (5xx) failures indicate invalid addresses, domain reputation issues, or authentication problems. If you recently increased your sending volume, you may have hit a rate limit, so consider scaling back temporarily. Additionally, verify whether your domain or IP is listed on a blocklist, as this can directly impact deliverability.
If your emails take forever to arrive, check if you’re sending too many too fast. Gmail, for example, often limits new IPs to 500 emails per hour for new IPs. Delays can also result from grey-listing, where emails are temporarily rejected before eventual delivery. Tweaking your DNS settings can help speed things up, making verification smoother and reducing delays over time.
Blocklists vary in severity, with some significantly impacting deliverability, such as the Spamhaus blocklist. Spamhaus maintains multiple lists, such as SBL, DBL, and ABL, which can immediately disrupt email delivery. Unlike Spamhaus, SpamCop’s SCBL list is user-reported and often resolves on its own if no further complaints arise. Other blocklists, like Barracuda (BRBL) and Proofpoint (Cloudmark CSI), affect corporate filters and some ISPs. Being listed on any blocklist is a clear signal that something is wrong, often due to outdated lists or sending without proper subscriber consent.
To check if you’re on a blocklist, use tools like MXToolbox. If you find your domain listed, visit the blocklist’s website to request removal, but first, make sure you have addressed and resolved any underlying issues that caused the listing in the first place.
Email authentication is key to building trust with ISPs and keeping spammers from pretending to be you. SPF confirms which servers can send on your behalf, DKIM makes sure your emails aren’t altered in transit, and DMARC brings it all together with policies and reporting. If you’re just getting started with DMARC, begin with a relaxed policy (p=none) to monitor activity, then tighten it up to p=quarantine or p=reject for stronger protection.
Maintaining list hygiene is essential. List hygiene is critical—regularly remove invalid and potentially dangerous emails (spam traps, bots, known complainers, etc.) and inactive or bounced addresses. If you’re using a new IP or domain, gradually increase your sending volume to build trust with ISPs and establish a positive reputation. Engagement-based segmentation can also help optimize deliverability by prioritizing active recipients while reducing or removing unengaged users. To maintain a consistent sender reputation, stick to a predictable sending schedule, immediately honor unsubscribe requests and spam complaints, and keep your bounce rate below 1% to avoid triggering ISP filters.
Are you trying to decide between a dedicated or shared IP? A dedicated IP gives you more control but also means you’re fully responsible for maintaining a good reputation and keeping deliverability high. While a shared IP benefits from the collective reputation of multiple senders, it can also be negatively affected by their practices. The correct IP choice will depend on your sending goals and needs.
Use a monitored reply-to address to handle responses and ensure your return-path domain matches your sending domain for better authentication. Additionally, implementing a one-click unsubscribe option is now required by Gmail, Yahoo, and Microsoft for bulk senders. Lastly, use specific subdomains for different types of emails, like marketing and transactional messages. This keeps one category from hurting the reputation of the other.
Monitoring open rates, bounce rates, and complaint trends can help identify issues early. Set up deliverability alerts to catch anomalies before they escalate, and don’t forget to conduct regular audits of your domain’s reputation and list quality.
By staying on top of your authentication, sending practices, and monitoring strategies, you can identify issues, improve email deliverability, and ensure that your messages reach the inbox instead of the spam folder.
Have questions? Get in touch.
John D. Bollinger is an email deliverability expert with over 20 years of experience helping brands maximize their ROI through ethical, data-driven email practices. He has led deliverability and compliance teams at leading ESPs and actively contributes to industry groups like M³AAWG and ESPC and is a frequent writer on email best practices. He loves dogs, enjoys cooking and BBQing, and spending time with family and friends.