You’ve spent hours crafting the perfect marketing email, but one bad link choice could send your carefully curated campaign straight to the spam folder—or worse, expose your subscribers to phishing attacks. Email link security is a balancing act between tracking performance and protecting your audience. Getting it wrong damages your sender reputation, erodes subscriber trust, and can even get your domain blacklisted.
In this guide, you’ll learn the essential link security practices for email marketing: how email security systems actually rewrite your links, why shortened URLs trigger spam filters, what authentication protocols you need to set up, and how to track performance without compromising safety. You’ll walk away with a clear action plan for keeping your email links secure and your campaigns out of spam folders.
TL;DR
- Email security services like Microsoft Defender and Proofpoint automatically rewrite every link in incoming emails, wrapping them with their own tracking prefixes for scanning . You cannot opt out of this—it happens regardless of your sender status.
- Shortened URLs are red flags for spam filters because they hide the destination. Avoid bit.ly, tinyurl, and similar services in marketing emails .
- Every link in your email acts as a “vote of confidence.” If the destination domain has a poor reputation, your own sender score suffers .
- Link tracking is essential for measuring performance, but it rewrites URLs in ways that make them look suspicious to recipients and security filters .
- SPF, DKIM, and DMARC authentication are non-negotiable for legitimate email sending. They prove to security systems that you are who you say you are .
Key Takeaways
- Email security platforms rewrite every link. Microsoft Defender rewrites URLs with prefixes like
nam01.safelinks.protection.outlook.com. Proofpoint usesurldefense.comprefixes . Recipients see these altered links, not your original URLs. - Rebranded tracking links build trust. Using
link.yourdomain.cominstead of a generic tracking domain shows recipients and spam filters that the link is legitimate . - Cold email campaigns should avoid links entirely in the first message. A permission-based approach—asking if you can share a resource—improves deliverability and engagement .
- Link reputation matters as much as sender reputation. Linking to low-reputation or blacklisted domains damages your deliverability through “guilt by association” .
- Consent banners and privacy laws complicate link tracking. Recipients who decline tracking cookies may still be counted as “opens” or “clicks,” making data unreliable .
Why Email Links Are Rewritten Automatically
When you send an email, the links you carefully crafted are often not the links your recipients actually see. Email security services rewrite every URL before the message reaches the inbox. This is not optional for senders—it’s a protection mechanism applied by the recipient’s email provider.
Microsoft Defender’s Safe Links feature rewrites URLs with a Microsoft prefix like https://nam01.safelinks.protection.outlook.com, followed by encoded parameters that preserve the original destination . This rewriting happens regardless of who sent the email—even trusted senders cannot bypass it .
Similarly, Proofpoint’s Targeted Attack Protection service rewrites links with https://urldefense.com/v1/url?u= prefixes. When a recipient clicks, Proofpoint checks the destination for malicious content before allowing the redirect . If the destination is harmful, the recipient sees a warning page instead of the website.
For senders, this rewriting has two practical consequences:
- Your carefully branded link appears as a long, unfamiliar URL, potentially reducing click-through rates
- Link-tracking services can interfere with these security rewrites, breaking your click measurement
How Link Choices Affect Deliverability
Spam filters don’t just evaluate your sender reputation—they evaluate every domain you link to. Each hyperlink in your email acts as a statement of trust. If you link to a domain with a poor reputation, your own deliverability suffers .
Here are the domain types that trigger spam filter suspicion:
Shortened URLs
Links from bit.ly, tinyurl, and similar services are high-risk. Spam filters distrust them because they hide the final destination, making it impossible to pre-scan the actual target . Even legitimate shortened URLs can slightly hurt deliverability, especially for high-volume senders .
Newly Registered Domains
Domains less than six months old lack trust history. Spam filters treat them like unknown entities and are more likely to flag emails containing them .
Free Hosting and File-Sharing Platforms
Services frequently abused by spammers—free image hosts, public file-sharing tools, and unauthenticated cloud storage—drag down the reputation of everyone linking to them .
Blacklisted Domains
If a domain appears on global blacklists like Spamhaus, URIBL, or SURBL, linking to it will almost certainly send your email to spam .
Securing Your Email Links: The Setup Steps
Step 1: Set Up Email Authentication (SPF, DKIM, DMARC)
Email authentication is the foundation of legitimate sending. Without SPF, DKIM, and DMARC records, your emails look suspicious to security systems regardless of your content .
For link security specifically, rebranded tracking links require CNAME records that map link.yourdomain.com to your email service provider’s tracking infrastructure. Most email platforms provide these records during setup .
Step 2: Rebrand Your Tracking Links
Instead of using your email platform’s default tracking domain (like link.mta3.com), configure link rebranding so tracked links appear as link.yourdomain.com . This does two things:
- Builds trust with recipients who recognize your domain
- Signals to spam filters that you’re a legitimate sender
The setup process involves adding CNAME records to your DNS. Your email platform will provide the specific values. Once verified, all tracked links in your emails will use your branded domain .
Step 3: Use HTTPS for All Links
Secure links (HTTPS) are non-negotiable. Security systems may block or flag emails containing insecure HTTP links . Ensuring all destination URLs use HTTPS is a prerequisite for maintaining trust with both email providers and recipients.
Step 4: Choose Destination Domains Carefully
Before including any link in your email, check the destination domain’s reputation. Use tools like:
- MXToolbox Blacklist Check to see if a domain appears on major blacklists
- URLVoid to analyze domains using multiple security engines
- VirusTotal’s URL scanner to check for malware or phishing flags
This is especially important for affiliate links, which often pass through intermediary domains. Each redirect in a chain adds risk—extra redirects look suspicious to spam filters .
The Problem with Link Tracking in Email
Email marketing platforms track clicks by rewriting each link with a tracking prefix. For example, a link to https://example.com might become https://discover.example.com/ab0cdefgh002trackyou, which then redirects to the original page .
This tracking creates security challenges:
First, it trains recipients to ignore URL mismatches. Security awareness training teaches people to hover over links and verify that the visible destination matches the actual URL. When tracking links always show different destinations, users become desensitized and may stop checking entirely .
Second, the “cloaked” URL is unfamiliar to recipients. A link that looks like https://discover.example.com/ab0cdefgh002trackyou appears suspicious, even though it’s legitimate tracking infrastructure .
Some organizations recommend turning off link tracking entirely, especially for internal or transactional emails, because the data loss is outweighed by security benefits. If click data is essential, it can be gathered through other tools like Google Analytics with properly tagged UTM parameters .
Special Considerations for Cold Email
Cold email campaigns require an even more cautious approach to links. Tracking links in cold emails can harm deliverability before you’ve established trust with the recipient.
A recommended approach is the “permission-based” strategy :
- Send the initial cold email without any links. Reference a resource or case study and ask if the recipient would like to see it.
- When the recipient replies with interest, send the link in your response.
- This creates a conversation, improves deliverability (no links in the initial message means maximum inbox placement), and qualifies interest before you share content .
For this approach, alternative tracking methods include using website analytics to track visits (if you send the link and they visit your site), focusing on reply rates and meetings booked instead of click-through rates, and setting up automation based on website visits rather than email clicks .
Comparison Table: Link Security Features
| Feature | What It Does | Why It Matters | Implementation Effort |
|---|---|---|---|
| Link Rebranding | Uses link.yourdomain.com instead of generic tracking domains | Builds trust; signals legitimacy to spam filters | Medium |
| HTTPS | Ensures all links use secure protocol | Required by security systems; prevents browser warnings | Low |
| SPF/DKIM/DMARC | Email authentication records | Proves sender legitimacy; required for deliverability | Medium |
| Destination Domain Checking | Screening URLs before linking | Prevents “guilt by association” deliverability damage | Low |
| Permission-Based Sending | Asking before sharing links in cold outreach | Maximizes deliverability; warms up conversations | Low |
Red Flags That Trigger Spam Filters
Spam filters evaluate links against these criteria. Avoid these patterns in your marketing emails:
- Shortened URLs: bit.ly, tinyurl, and similar services are automatically suspect
- Mismatched display text and URL: The visible text says “Google” but the link goes elsewhere
- Redirect chains: Multiple redirects before reaching the destination
- Domains with spam history: Check each destination domain’s reputation before linking
- New or unknown domains: Domains less than six months old lack trust
Frequently Asked Questions
Can I stop Microsoft Defender or Proofpoint from rewriting my links?
No. You cannot bypass Safe Links or similar security services. These protections are applied by the recipient’s email provider and cannot be disabled, even for trusted senders .
Are shortened URLs safe to use in email marketing?
No. URL shorteners are red flags for spam filters and damage deliverability. They are also frequently exploited by attackers to hide malicious destinations . Use clean, full URLs instead.
How do I check if a destination domain is safe?
Use tools like MXToolbox Blacklist Check, URLVoid, or VirusTotal to scan domains for blacklist status and malware reports before including them in your emails .
Does link rebranding improve deliverability?
Yes. Using a branded domain for tracked links (like link.yourdomain.com) signals legitimacy to spam filters and builds trust with recipients. It also prevents your emails from appearing like they come from a third-party tracking service .
Should I track clicks in cold emails?
No. Tracking links in cold emails harm deliverability. Instead, use a permission-based approach: reference a resource in the first email without a link, and only send the link after the recipient expresses interest .
What’s the safest way to include affiliate links in emails?
Use full, clean URLs with your own domain if possible. Avoid redirect chains and shortened URLs. Check the affiliate domain’s reputation before linking. Consider using a branded link management platform that preserves your domain in the tracking link .
Sources
- Constant Contact: Link Rebranding and CNAME Setup
- University of Washington: Mass Email Link Tracking Best Practices
- Microsoft Learn: Safe Links and Sender Best Practices
- Microsoft Learn: Safe Links Policy Configuration
- Dialog Insight: Why Emails Are Flagged as Phishing
- Lemlist: Cold Email Link Tracking Best Practices
- SmartReach: Email Links and Domain Reputation
- University of Illinois: Proofpoint URL Defense Information
- Stony Brook University: Identifying Illegitimate Email Links
Have you experienced deliverability issues from link choices in your email campaigns? Share your experience in the comments and help others learn from your challenges.
