Summary
Emails bouncing with a "550 Rejected by header based Anti-Spoofing policy" message from Mimecast indicate that the recipient's Mimecast security gateway is identifying your emails as potentially spoofed or unauthorized. This often happens when an email appears to originate from an internal domain but is sent via an external service provider, leading Mimecast to suspect an attempt at impersonation. Even if your standard email authentication protocols like SPF, DKIM, and DMARC are correctly configured, Mimecast's internal policies can still flag the email. The core issue usually lies in Mimecast's perception of emails originating from a domain it manages, but arriving from an unexpected external source.
Key findings
- Internal spoofing detection: Mimecast's anti-spoofing policies are designed to block emails that appear to come from an internal domain but are sent from an external (non-Mimecast) server.
- Authentication layers: Even with correct SPF and DMARC alignment, Mimecast may still reject messages based on its proprietary header-based anti-spoofing.
- Pre-queue rejection: These bounces occur very early in the mail flow, often before the message even reaches the recipient's internal mail queue within Mimecast, making them difficult to troubleshoot without direct Mimecast access.
- EHLO domain SPF check: Some Mimecast configurations might perform an SPF check on the EHLO (MAIL FROM) domain rather than the Return-Path domain, which can cause issues if not properly aligned.
Key considerations
- Mimecast policy modification: The most direct solution typically involves configuring Mimecast's anti-spoofing policy to allow the specific sender's address or IP address. This usually means creating an exception within Mimecast's policy settings. For more information, refer to Mimecast's support documentation on anti-spoofing policies.
- Communication with recipient: Since the issue resides with the recipient's Mimecast setup, direct communication with their IT team is essential to implement the necessary allowlist (or whitelist) changes or policy adjustments.
- Sender address and IP configuration: Ensure that the sending IP address and domain are consistent and properly authorized in your own SPF records. Also, check your SMTP HELO/EHLO hostname matches your brand domain, as suggested by HeightOn Agency.
- Subdomain handling: If you are sending from a subdomain of the recipient's primary domain, Mimecast is highly likely to flag it as spoofing, especially if it's handled externally. Allowlisting is critical in such scenarios.
What email marketers say
Email marketers frequently encounter Mimecast's anti-spoofing policies when sending emails that, despite being legitimate, appear suspicious to the system. This often arises when external sending platforms are used for domains that Mimecast manages internally. The primary challenge for marketers is navigating these security measures without direct control over the recipient's Mimecast configuration, necessitating close collaboration with the recipient's IT team.
Key opinions
- Sender-side authentication is not always enough: Even with proper SPF and DMARC setup, Mimecast's internal policies can still cause bounces, highlighting the need for recipient-side adjustments.
- External sending of internal domains: A common scenario involves an organization sending emails for their own domain via a third-party ESP (Email Service Provider), which Mimecast perceives as spoofing.
- Policy overrides are crucial: Many believe the simplest solution is for the recipient's IT department to create a Mimecast policy override or allowlist for the sending IP address or domain.
- Testing is key: Thorough testing to an internal recipient is often needed to confirm that the Mimecast policies are correctly configured to prevent false positives.
Key considerations
- Recipient collaboration: Successful resolution heavily depends on the recipient's willingness and ability to adjust their Mimecast settings.
- Verify SPF on EHLO domain: If a bounce refers to a header-based policy, checking that the SPF record properly validates the EHLO domain (not just the Return-Path) can be a critical step.
- Consider backscatter: While not directly related to this specific bounce, Mimecast's policies aim to prevent issues like email backscatter, where spammers spoof your address and bounces return to you.
- Review MTA configuration: If sending to a test server within the same network, confirm the MTA is presenting the expected public IP, not a NAT IP, which could confuse Mimecast.
Email marketer from Email Geeks explains that the core challenge often lies in figuring out what built-in header policies Mimecast could have that would reject emails before they even reach the customer's queue. The available Mimecast documentation might not always provide immediate solutions, making direct investigation difficult.
Email marketer from Email Geeks suggests that even if standard authorization headers are correct and there isn't a strict DMARC policy, the issue persists. They mention trying to ensure SPF alignment and attempting a policy override in Mimecast, though these actions may not always resolve the problem if messages don't appear to be reaching Mimecast at all.
What the experts say
Email deliverability experts recognize Mimecast's anti-spoofing as a sophisticated layer of defense, often catching legitimate emails that deviate from expected mail flow patterns. While robust, these policies require precise configuration to avoid blocking valid senders. Experts emphasize the importance of understanding not just standard email authentication but also how specific security gateways like Mimecast interpret and enforce sender identity, especially for internal domains sending externally.
Key opinions
- Mimecast's unique checks: Mimecast goes beyond standard SPF/DKIM/DMARC by implementing its own header-based checks that specifically target apparent internal spoofing.
- Internal domain sensitivity: Any email originating from a company's domain but arriving from an external IP not explicitly known to Mimecast will be scrutinized heavily.
- Sender reputation vs. policy: These bounces are often policy-driven rather than reputation-driven. A good sender reputation won't override a conflicting Mimecast anti-spoofing policy.
- Proactive allowlisting: For organizations using Mimecast, it's best practice to proactively allowlist all legitimate external senders that send on behalf of their domain, including transactional or marketing ESPs.
Key considerations
- Comprehensive authentication: Ensure that your entire email ecosystem (ESPs, CRMs, internal systems) is properly authenticated with robust SPF, DKIM, and DMARC records to lay a strong foundation, which can be monitored with DMARC monitoring.
- Understanding return-path vs. from: Be aware that different security solutions may check different parts of the email header for authentication, and Mimecast often scrutinizes the From header more closely for spoofing.
- IP address consistency: Maintain consistent sending IP addresses and ensure they are covered by SPF. If you use shared IP addresses, ensure your ESP has proper domain-level authentication methods in place.
- Recipient-specific configurations: Recognize that deliverability issues with Mimecast often require direct intervention or configuration changes by the recipient's IT team. This means effective communication is paramount to resolving these types of bounces, which are quite distinct from other recipient-specific rejections.
Email expert from Word to the Wise emphasizes that domain alignment is critical for preventing spoofing alerts. They note that many anti-spoofing systems specifically check for discrepancies between the visible "From" address and the underlying sending domain, blocking messages when these don't align properly.
Email expert from Spam Resource states that overly aggressive anti-spoofing policies can lead to legitimate mail being blocked. They advise administrators to carefully balance security with usability, often through the strategic implementation of policy exceptions for known and trusted external senders.
What the documentation says
Mimecast's official documentation highlights that its anti-spoofing policies are a crucial component of its email security architecture, specifically designed to combat internal and direct domain spoofing. These policies analyze email headers to determine if a message claiming to be from an internal domain is indeed originating from an authorized source. The documentation provides clear guidelines for administrators on how to configure, review, and create exceptions within these policies to ensure legitimate email flow while maintaining robust security.
Key findings
- Policy configuration: Mimecast allows administrators to create and manage anti-spoofing policies through its administration console.
- Internal domain protection: The primary goal is to protect against emails that falsify the internal domain (header spoofing), preventing phishing and impersonation attacks.
- Sender-based exceptions: Exceptions can be made based on the sender's email address or IP address, allowing legitimate external senders using the internal domain to bypass the anti-spoofing checks.
- Policy narrative: Policies can include a narrative to explain their purpose, aiding in management and troubleshooting.
Key considerations
- Applying policy overrides: Administrators must explicitly create policies to override the default anti-spoofing behavior for specific legitimate traffic, such as emails from a marketing automation platform or a CRM system. These are critical in situations like receiving bounce messages for emails you didn't send.
- Excluding Mimecast IPs: Many default anti-spoofing policies are set to exclude Mimecast's own IPs, ensuring that mail flowing through their service is not erroneously flagged.
- Reviewing existing policies: Before creating new policies, review existing anti-spoofing policies to understand their scope and order of application, as policy conflicts can occur.
- Domain vs. subdomain handling: Special attention should be paid to how Mimecast treats subdomains, as an external sender using a subdomain can still trigger anti-spoofing if not explicitly allowed, such as when dealing with personal emails from a custom domain.
Mimecast's documentation for "Policies - Configuring Anti-Spoofing" explains that anti-spoofing policies are essential for blocking unwanted spoof emails and protecting internal domains. It details how administrators can configure these policies to allow legitimate senders while preventing malicious attempts to impersonate internal users or domains. This involves setting specific actions for detected spoofing attempts.
The Zendesk article on Configuring Anti-Spoofing Policies for Mimecast outlines that to address messages triggering an anti-spoofing policy, administrators should create an Anti-Spoofing policy. This policy should be configured to take "no action" for the sender's address or IP address, effectively creating an exception for legitimate mail flow that would otherwise be blocked.
