Why are routine emails with third-party advertisements getting spam blocked?
Michael Ko
Co-founder & CEO, Suped
Published 9 Jul 2025
Updated 19 Aug 2025
6 min read
It can be incredibly frustrating to find that routine emails, especially those containing third-party advertisements, are consistently being flagged as spam. You're sending messages to an engaged audience, perhaps even paying subscribers, yet they aren't reaching the inbox.
This issue is complex because multiple factors influence whether an email lands in the inbox or the spam folder. When third-party advertisements are involved, an additional layer of complexity is introduced, often tying your email's fate to the reputation of external entities.
I've seen many legitimate senders face this challenge, even when sending daily, consistent emails. The key is to understand the various elements at play, from sender reputation to content filtering and subscriber engagement.
The shared reputation risk
One of the most significant reasons routine emails with third-party advertisements get spam blocked is the concept of shared reputation. When you include links, images, or content hosted by a third-party ad provider, your email's deliverability becomes intertwined with that provider's sending history and reputation.
If that third-party provider also works with senders who engage in questionable practices, or if their domains frequently appear in emails marked as spam, it can negatively impact your email, even if your own sending practices are impeccable. Internet service providers (ISPs) and email clients often cast a wide net, blocking or filtering emails associated with known problematic entities.
This means a legitimate email with an ad from a previously compromised domain could inadvertently trigger spam filters, regardless of your sender score. It's crucial to thoroughly vet any third-party ad partners and continuously monitor their reputation.
Vetting ad partners
Due Diligence: Research the ad network's reputation and any history of spam complaints or blocklist (blacklist) appearances.
Domain and IP Health: Check the domains and IP addresses used for their ads against public blocklists (or blacklists). Consider if your domain reputation is at risk.
Transparency: Ensure clear unsubscribe options and compliance with regulations like the CAN-SPAM Act.
Ignoring ad partner reputation
Reputation Damage: Your sender reputation can suffer significantly if linked to shady ad providers, leading to broader deliverability issues.
Increased Blocks: Emails with ads from blacklisted (or blocklisted) domains are highly likely to be blocked, even if your content is otherwise benign.
Loss of Trust: Subscribers may lose trust in your emails if they consistently find themselves in the spam folder due to third-party content.
Content and user engagement
The content within your emails, especially the advertisements, plays a significant role in spam filtering. Spam filters are increasingly sophisticated, analyzing everything from keywords and image ratios to embedded links and HTML structure. Ads, by their nature, often contain elements that can trigger these filters, such as excessive promotional language, suspicious links, or certain image formats.
Furthermore, user engagement metrics heavily influence how ISPs perceive your emails. If recipients consistently mark your emails as spam, even if the primary content is legitimate, it signals to ISPs that your messages are unwanted. This is especially true if you are sending marketing emails to recipients who aren't interested in the advertisements. The more people who mark emails as spam, the more likely future emails are to be filtered into the spam folder.
Even for routine, low-volume sends to paid subscribers, if the third-party ads within them cause a spike in complaints, it can lead to blocks from specific domains or ISPs. It's a subtle but powerful signal that your content, or parts of it, are undesirable.
Content optimization and engagement
Balance Content: Ensure your primary email content outweighs the advertising material.
Ad Placement: Consider placing ads strategically to minimize their impact on the main message.
Link Hygiene: Only include links to reputable, well-known domains. Avoid shortened or obscure URLs.
Engagement Monitoring: Track open rates, click-through rates, and, crucially, spam complaint rates to gauge subscriber reaction.
Technical foundations for deliverability
Even with perfect content and trusted ad partners, technical misconfigurations can lead to spam blocking. Your email authentication protocols, such as SPF, DKIM, and DMARC, are fundamental to deliverability. If these are not correctly configured, ISPs may view your emails with suspicion, especially when they originate from a third-party email service provider (ESP) or contain external links.
For instance, an SPF record that doesn't include the IP addresses of your ad server or a DKIM signature that breaks due to ad content modification can lead to authentication failures. These failures, even for routine transactional emails, can cause them to be filtered as spam. It's a common reason why emails sent via a third-party ESP are flagged. Regular validation of these records is essential.
Furthermore, if your own sending IP address or domain gets listed on a public blacklist (or blocklist), it will severely impact deliverability, regardless of the content. While third-party ads might not directly cause your domain to be blacklisted, their presence can exacerbate the problem if your overall email practices are not up to par. It's vital to monitor your email blacklists and blocklists continuously.
Missing or misconfigured SPF, DKIM, DMARC records.
Authentication failures, outright rejections, or messages ending up in Yahoo spam.
Proactive monitoring and remediation
Proactive monitoring is your best defense against routine emails getting spam blocked. This involves more than just checking your bounce messages; it requires a deep dive into your email logs to understand the precise reasons for rejections. A generic "spam" bounce code provides little actionable information, so demand more detailed feedback from your ESP or look into raw logs yourself.
Regularly checking your domain's health and looking for listings on email blocklists (or blacklists) is paramount. If you notice issues, investigate the specific URLs or domains used in your third-party ads to see if they are the culprits. Removing problematic ads or working with more reputable partners can yield immediate improvements in deliverability, as demonstrated by many senders who isolate and resolve these issues.
Example of a specific bounce messageplain
550 5.7.1 Service unavailable; Client host [192.0.2.1] blocked using [List.SpamBlocklist.com]; Client [192.0.2.1] listed on Blocklist, see http://www.spamblocklist.com/lookup?ip=192.0.2.1
Additionally, consider implementing robust email authentication, such as DMARC, SPF, and DKIM, and closely monitor your DMARC reports. These reports provide invaluable insights into how your emails are being authenticated and handled by receiving servers, helping you pinpoint issues related to third-party content or sending infrastructure.
Views from the trenches
Best practices
Actively monitor specific bounce messages, not just simplified 'spam' notifications, to diagnose issues.
Rigorously vet all third-party ad providers for any signs of problematic sending or domain history.
Ensure all email authentication protocols, including SPF, DKIM, and DMARC, are correctly configured.
Periodically review email content for spammy keywords or link structures, especially in advertisements.
Prioritize subscriber engagement by ensuring content remains valuable and offering clear unsubscribe options.
Common pitfalls
Generic bounce messages hiding specific issues like IP or domain blocklisting (blacklisting).
Underestimating the impact of a third-party ad domain's poor reputation on your own deliverability.
Assuming consistent email content guarantees deliverability, ignoring dynamic factors like ad links.
Neglecting to monitor user spam complaints, which significantly influence sender reputation.
Failing to adapt to evolving ISP spam filtering techniques and authentication requirements.
Expert tips
Continuously check your sending IPs and domains against major public blocklists (blacklists).
Use a DMARC monitoring service to gain granular visibility into authentication failures and delivery.
Regularly test your email campaigns with an email deliverability tester to predict inbox placement.
Segment your audience and tailor ad content to minimize irrelevant promotions and reduce complaints.
Maintain a clean mailing list by regularly removing disengaged subscribers and invalid email addresses.
Marketer view
Marketer from Email Geeks says they started seeing spam blocks on routine emails with third-party ads after October 9th, despite no changes to the email's core content or sender practices. The bounce messages were vague, simply stating 'spam'.
2019-10-18 - Email Geeks
Marketer view
Marketer from Email Geeks says they found one of the third-party advertisement domains included in their emails to be highly suspicious after checking it with domain reputation tools. This correlated with the onset of their spam blocking issues.
2019-10-18 - Email Geeks
Maintaining deliverability for ad-inclusive emails
Dealing with routine emails getting spam blocked, especially when they include third-party advertisements, requires a multifaceted approach. It’s rarely a single issue but rather a combination of factors related to sender reputation, content, and technical configurations.
By diligently vetting ad partners, optimizing your email content, ensuring robust email authentication, and maintaining vigilant monitoring, you can significantly improve your email deliverability and ensure your messages consistently reach the intended inboxes.
