What specifically do ISPs check when a third-party email links to my site?

ISPs check the sender's IP and domain reputation, email authentication (SPF, DKIM, DMARC), and content quality. If any of these elements are flagged, even with a valid link to your site, the email can be rejected. Your site's reputation is also considered within the context of the sending email.

Does the third party's email sending reputation matter if the link is to my reputable domain?

Yes, their sending practices directly impact deliverability. A third party's low sender reputation, high spam complaints, or being listed on a blacklist (or blocklist) will cause rejections, even if your linked domain has a good reputation. This is why it's important to monitor how third-party links in email affect deliverability .

How can I troubleshoot link-related issues in third-party emails?

Check that the third party has properly configured SPF, DKIM, and DMARC records that align with your domain if they are sending on your behalf. Also, examine the link structure in the email for any discrepancies between the display URL and the actual URL, or unusual characters that could flag it as suspicious.

What content or engagement factors might cause rejections?

Ensure your partners maintain clean mailing lists, avoid sending to spam traps, and encourage high recipient engagement. Advise them to use clear, non-spammy content and to avoid excessive or deceptive links to help prevent emails from landing in spam.

Why are third-party emails linking to my website getting rejected by ISPs? - Sender reputation - Email deliverability - Knowledge base

It can be incredibly frustrating when emails from your trusted third-party partners are rejected by Internet Service Providers (ISPs), especially when those emails link to your website. You might be sending your own emails successfully, so it feels like a double standard. The primary reason this happens is that ISPs evaluate every email's journey and its contents, not just your direct sending practices.

When a third party sends an email mentioning or linking to your domain, your domain's reputation becomes intertwined with theirs. Even if your own email program has stellar deliverability, the third party's sender reputation, authentication setup, and email content can trigger spam filters, leading to rejections or messages landing in the junk folder.

This scenario highlights a crucial aspect of email deliverability: it's a shared responsibility. While you control your own sending practices, understanding the factors that affect third-party emails referencing your site is key to maintaining your brand's integrity and ensuring your content reaches its intended audience.

The impact of sender reputation

One of the most significant factors influencing whether an email gets delivered is the sender's reputation. When a third party sends an email, their sending IP address and domain are what the ISP primarily evaluates. If their sending reputation is poor, it can lead to rejections, regardless of the content or the reputation of the domains linked within the email.

This poor reputation often stems from issues like high spam complaints, sending to old or invalid email addresses, or being listed on various email blocklists (or blacklists). Many ISPs use their own internal reputation services, and if the third party has a low score there, emails will be blocked. Even major ISPs like

Google track sender practices, and a poor history can lead to immediate rejections.

To safeguard your own domain's reputation, it's vital that any third parties sending emails that link to your site maintain excellent sending practices. This includes regular list hygiene, monitoring their own complaint rates, and avoiding common blocklists. You can learn more about how email blacklists work and how they can affect deliverability.

Some blocklists specifically target IP addresses from which email should never be sent directly, such as The Spamhaus Policy Blocklist (PBL). If your partner is sending from an IP on such a list, it's a clear red flag for receiving mail servers. Understanding these nuances can help you identify why common blocklists cause message rejected errors.

Authentication and link configuration

Email authentication protocols such as SPF, DKIM, and DMARC are fundamental for verifying a sender's legitimacy. When a third party sends emails on your behalf or includes links to your domain, proper authentication is critical. If their setup isn't aligned, ISPs can view the email as suspicious or even a spoofing attempt, leading to rejection.

For instance, an email might display your domain in the 'From' address, but the underlying sending domain (the one authenticated by SPF or DKIM) might belong to the third party. If DMARC isn't properly configured to allow this relationship, it can result in a DMARC verification failed error. This mismatch often triggers strong spam filters.

Beyond authentication, the links themselves can be problematic. Issues like mismatched visible text and actual link URLs (e.g., a link that says "yourdomain.com" but points to a different tracking domain), excessive numbers of links, or even suspicious-looking URL structures (like yourdomain-com instead of yourdomain.com) can raise red flags for spam filters. ISPs are constantly looking for phishing attempts, and deceptive links are a common tactic.

It's important to establish clear guidelines with third parties regarding link usage and to ensure they have robust email authentication in place. A simple guide to DMARC, SPF, and DKIM can help them understand the requirements for proper alignment.

Common link issues

Mismatched domains: When the visible link text differs significantly from the actual destination URL.
Excessive links: Emails with too many links can appear spammy to filters.
URL shorteners: While convenient, they can be associated with malicious activity and raise flags.
Malware/phishing patterns: URLs that mimic legitimate domains (typosquatting) or contain suspicious characters.

Content and recipient engagement factors

Even with perfect authentication and link structure, the content of the email itself can lead to rejections or spam folder placement. ISPs analyze email content for characteristics commonly associated with spam, such as overly promotional language, certain keywords, or poor formatting.

If the third-party email is perceived as low-quality or unsolicited, even if it contains a perfectly legitimate link to your website, it can be flagged. This is why sender score is a crucial metric, as it reflects the overall quality and trustworthiness of a sender's emails.

Recipient engagement also plays a huge role. If the third party's subscribers frequently mark their emails as spam, or simply ignore them (leading to low open and click rates), their overall reputation suffers. ISPs interpret low engagement and high complaint rates as indicators of unwanted mail, which then negatively impacts all emails from that sender, including those linking to your site.

ISP policies and network dynamics

Every ISP has its own unique filtering policies and algorithms, which constantly evolve. What passes through one ISP's filters might be aggressively blocked by another. This can lead to situations where emails are rejected by some ISPs but delivered successfully by others. Small ISPs, in particular, may have stricter or less sophisticated filtering mechanisms.

Sometimes, the issue isn't directly related to your domain or the third party's sending practices but rather the IP address they are using. If the third party's email service provider (ESP) uses shared IP addresses, and another sender on that same IP sends spam, it can lead to the entire IP (and thus your partner's emails) being blocklisted. This is a common reason why legitimate emails might be blacklisted despite sending legitimate email.

Furthermore, residential IP addresses are often blocked from sending direct email due to their association with spam bots and compromised machines. If a third party is sending emails via a setup that looks like a residential connection, ISPs may automatically reject those emails. This can be especially true for ISPs like Optonline or

Spectrum, which are known for strict policies.

Given the complexity, diagnosing these rejections can be challenging. It requires careful analysis of bounce messages and potentially direct communication with the third party's email team to understand their sending infrastructure and reputation. In some cases, it might even require reaching out to the specific ISP postmaster for clarification, as seen with some sudden rejections by Gmail.

Views from the trenches

Best practices

Ensure third-party senders have robust email authentication (SPF, DKIM, DMARC) properly configured and aligned with your domain for maximum deliverability.

Regularly monitor your own domain's reputation using tools like Google Postmaster Tools and encourage partners to do the same for their sending domains.

Advise third parties to maintain clean email lists and avoid sending to unengaged or old contacts to minimize spam complaints and maintain a healthy sender reputation.

Collaborate with partners to review email content and link practices, ensuring they avoid spammy triggers, excessive links, or deceptive link formatting.

Common pitfalls

Assuming your domain's good reputation automatically extends to emails sent by third parties, as ISPs evaluate the actual sender's reputation.

Ignoring the specific bounce messages received, which often contain clues about why the email was rejected (e.g., blocklist mentions, authentication failures).

Failing to communicate clear email sending guidelines and authentication requirements to partners who link to your website.

Overlooking deceptive link practices, such as mismatched display URLs and actual target URLs, which can trigger fraud filters.

Expert tips

Always advise partners to use dedicated subdomains for their email sending, if possible, to isolate any reputation issues from their main domain.

Implement a DMARC policy with reporting (p=none) to monitor how third parties are sending emails on your behalf and identify any authentication failures.

Conduct periodic email deliverability tests from your partners' sending environments to proactively identify potential rejection issues with links to your site.

Educate partners on the importance of email engagement metrics; low opens and high spam complaints directly impact sender reputation for linked content.

Expert view

Expert from Email Geeks says a common issue is when the visible link text in an email differs from the actual URL. This can trigger fraud warnings from ISPs.

2024-03-10 - Email Geeks

Marketer view

Marketer from Email Geeks says they found their partner's emails were being rejected because of suspicious URL patterns like using a dash instead of a dot in a domain-like segment, which looked like phishing.

2023-11-20 - Email Geeks

Navigating deliverability challenges

The rejection of third-party emails linking to your website can be traced back to several factors, often a combination of the third party's sender reputation, their email authentication practices, and the content and structure of their emails. ISPs are diligent in protecting their users from unwanted or malicious content, and any red flag, even a subtle one, can lead to a block.

Proactive measures, such as ensuring your partners adhere to strong deliverability best practices and properly authenticate their emails when sending on your behalf, are essential. Regularly reviewing bounce messages for specific rejection reasons and using tools to monitor your domain and their sending IPs can help you pinpoint issues quickly. This allows for effective communication and troubleshooting to ensure that all legitimate emails, regardless of who sends them, reach the inbox.