When should domains be allowlisted for email sending?
Matthew Whittaker
Co-founder & CTO, Suped
Published 30 May 2025
Updated 16 Aug 2025
6 min read
The question of when to allowlist (or whitelist) domains for email sending is complex, often misunderstood, and highly nuanced. At its core, allowlisting is intended to ensure that emails from specific, trusted sources bypass spam filters and reach the inbox. It sounds like a straightforward solution to deliverability challenges, but the reality is far more intricate in today's email security landscape.
Relying on broad allowlists can introduce significant security vulnerabilities, potentially undermining the very protections you aim to establish. Modern email security prioritizes robust authentication over simple lists of approved senders. Understanding this shift is critical for maintaining both deliverability and a strong security posture.
The evolving landscape of email trust
Historically, broad allowlisting of IP addresses or domains was a common practice to ensure email deliverability. This approach meant that an email server would automatically accept mail from a specified source, bypassing standard spam and security checks. It was a simple way to guarantee that emails from known entities would always arrive.
Today, this method is largely outdated and carries substantial risks. Modern email security heavily relies on robust authentication protocols such as SPF, DKIM, and DMARC. These protocols work together to verify the legitimacy of the sender and the integrity of the message, moving beyond a reliance on static lists of "trusted" sources.
Blindly allowlisting domains without proper authentication mechanisms in place can inadvertently open your organization to phishing and spoofing attacks. If a malicious actor spoofs a domain you have broadly allowlisted (or whitelisted), their illegitimate emails could bypass your security measures and land directly in users' inboxes, posing a major threat.
Potential for spoofing and phishing
Broad allowlisting means that any email claiming to be from an allowlisted domain, regardless of its true origin or authentication status, could bypass your email security. This creates a severe vulnerability for your organization, making it susceptible to sophisticated phishing attacks, business email compromise (BEC), and other forms of email fraud.
When targeted allowlisting is still a necessity
Despite the industry's shift towards authentication, there are still specific, limited scenarios where allowlisting (or whitelisting) a domain can be necessary. These instances typically involve internal systems, tightly controlled B2B communications, or critical transactional services where deliverability is paramount and the sender is explicitly known and trusted.
For example, some internal applications or legacy systems might send automated alerts that are occasionally flagged by aggressive spam filters. In such cases, carefully allowlisting the specific sending domain for these internal communications might be required to ensure crucial messages are always received by your team.
Another scenario involves highly regulated industries or close business partnerships where a partner's unique email sending infrastructure occasionally triggers false positives. Here, a mutual agreement to specifically allowlist their verified sending domains might be a short-term solution, provided both parties maintain strict authentication.
Outdated or risky scenarios
Mass marketing emails: Using broad allowlists for marketing campaigns.
Public domains: Whitelisting generic domains such as gmail.com or outlook.com.
Unauthenticated senders: Allowing domains without SPF, DKIM, or DMARC records.
Valid, targeted scenarios
Internal systems: For critical alerts from internal applications.
Specific B2B partners: In highly controlled, verified environments.
Critical transactional alerts: From trusted, essential third-party services.
The critical role of email authentication
When considering any form of allowlisting (or whitelisting), it is imperative that the domains in question are fully authenticated. This means they must have correctly configured SPF, DKIM, and DMARC records. These email authentication standards are the foundation of modern email security and deliverability, verifying sender legitimacy.
SPF (Sender Policy Framework) allows a domain owner to specify which mail servers are authorized to send email on their behalf. DKIM (DomainKeys Identified Mail) provides a way for email senders to cryptographically sign their emails, verifying that the message has not been tampered with in transit. Both are crucial for establishing trust.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM by providing a framework for email receivers to authenticate incoming mail. It dictates what should happen to emails that fail SPF or DKIM checks and provides reporting back to the domain owner. For an allowlist to be truly secure, the domain should have a valid DMARC policy with alignment. You can learn more about these protocols in a simple guide to DMARC, SPF, and DKIM.
If you absolutely must allowlist (or whitelist) a domain, precision is paramount. Avoid using broad wildcards like *.yourdomain.com if possible, as this can inadvertently allow mail from unauthenticated or malicious subdomains. Instead, explicitly list each subdomain that needs to be allowlisted, such as support.company.com or account.company.com. Consider when to use subdomains for email sending for your organization.
Regular review of your allowlists (and blocklists) is also crucial. Business relationships change, and third-party vendors update their sending infrastructure. What was safe to allowlist yesterday might not be today. Maintaining a clean and current allowlist reduces your risk exposure and helps maintain a robust security posture. Consider also reviewing how to resolve domain blocklisting and improve email reputation to understand the inverse of allowlisting.
Views from the trenches
Best practices
Always require domains to be fully authenticated with DMARC, SPF, and DKIM before allowing them.
Be specific about subdomains when allowlisting, avoiding broad wildcards unless strictly necessary.
Regularly audit and review your allowlist entries to ensure they are still valid and secure.
Common pitfalls
Allowlisting public domains, which significantly increases the risk of spoofing and phishing.
Creating blanket allowlists that bypass all spam filtering without authentication checks.
Failing to update allowlists when senders change their IP addresses or sending domains.
Expert tips
Prioritize strong email authentication and monitoring tools over extensive allowlists for overall security.
Educate clients and partners on the importance of implementing DMARC policies themselves.
Use allowlists only for critical, known, and secure internal or trusted partner communications.
Expert view
Expert from Email Geeks says: They must absolutely not whitelist domains unless they are fully authenticated and verified.
April 1, 2023 - Email Geeks
Expert view
Expert from Email Geeks says: Otherwise, any spoofing of your domains would be seen as legit.
April 1, 2023 - Email Geeks
A balanced approach to email security
In summary, allowlisting (or whitelisting) domains for email sending is a tool that should be used with extreme caution and only in very specific circumstances. It is not a substitute for proper email authentication, which remains the cornerstone of email deliverability and security. Understanding why your emails go to spam is key.
The primary goal should always be to ensure your emails are properly authenticated and that your sending practices build a strong email sending domain reputation. If deliverability issues arise, look first to your SPF, DKIM, and DMARC configurations, rather than defaulting to broad allowlists. This approach not only improves inbox placement but also protects your recipients from malicious emails and maintains the integrity of your brand.
