Summary
Emails are bouncing due to Mimecast Anti-Spoofing policies primarily due to misconfiguration or overly aggressive rules. Root causes include: SPF record issues (missing sources, EHLO domain failures), incorrect DMARC settings, unauthenticated internal IPs, subdomain problems, unvalidated email headers, issues with third-party senders, and Mimecast's own header inspection rules. Solutions involve: creating exception policies, allowlisting trusted senders, ensuring proper SPF/DKIM/DMARC configuration, regularly reviewing Mimecast logs, validating email headers, and collaborating with Mimecast support.
Key findings
- SPF Issues: Incomplete or incorrect SPF records, specifically regarding EHLO domains and authorized sending sources, trigger Anti-Spoofing policies.
- DMARC Misconfiguration: Strict DMARC policies combined with SPF and DKIM failures cause emails to be rejected.
- Internal IP Authentication: Mimecast blocks emails from internal servers when their IPs are not recognized or trusted.
- Subdomain Problems: Improperly configured subdomains can cause bounces if Mimecast doesn't recognize them as part of the main domain's trusted sources.
- Third-Party Vendor Issues: Emails sent via third-party vendors get rejected if permissions are missing or the vendor isn't listed as an allowed sender.
- Header Inspection Rules: Emails are rejected if the mail headers do not meet the Mimecast's header inspection rules, even with the existence of SPF, DKIM, and DMARC.
- Aggressive Policies: Mimecast is known for having aggressive Anti-Spoofing policies that can mistakenly flag legitimate emails.
Key considerations
- Exception Policies: Create Anti-Spoofing policies in Mimecast to exclude legitimate senders based on their IP address, email address, or domain.
- SPF Record Accuracy: Regularly check and update SPF records to include all valid sending sources, paying close attention to the EHLO domain.
- DMARC Alignment: Ensure DMARC settings align with SPF and DKIM, and understand the consequences of strict DMARC policies.
- Mimecast Allowlisting: Use the allowlisting feature to explicitly trust internal servers and authorized third-party senders.
- Log Analysis: Utilize Mimecast's message tracking logs to identify the cause of email rejections and fine-tune policies.
- Collaboration with Mimecast Support: Work closely with Mimecast support to troubleshoot issues and optimize policy settings.
- Proper Header Validation: Ensure all outgoing email messages have the proper headers to avoid anti-spoofing filters.
What email marketers say
10 marketer opinions
Emails bounce with Mimecast Anti-Spoofing policies due to various reasons including SPF record issues, misconfigured DMARC settings, internal IPs not recognized, subdomain issues, lack of proper authentication for internal emails, third-party vendor problems, and Mimecast's own aggressive policies and header inspection rules. Resolution involves allowlisting, verifying SPF records including EHLO domains, configuring DMARC/DKIM records properly, adding sending server IPs to Mimecast, and reviewing Mimecast logs.
Key opinions
- SPF issues: Incorrect or incomplete SPF records, especially those not including EHLO domains or all authorized sending sources, can trigger Anti-Spoofing policies.
- DMARC misconfiguration: Strict DMARC policies ('reject' or 'quarantine') combined with SPF/DKIM failures lead to rejected emails.
- Internal IP recognition: Mimecast may block emails from internal servers if the server's IP address isn't recognized or trusted.
- Subdomain issues: Improperly configured subdomains without corresponding SPF/DKIM records can cause bounces.
- Authentication problems: Lack of proper SPF, DKIM, and DMARC setup for internal emails leads to emails being flagged as spoofed.
- Third-party vendor issues: Third-party vendors sending emails without proper permissions or being listed as authorized senders results in rejections.
- Mimecast Aggressiveness: Mimecast's aggressive Anti-Spoofing policies, without proper configuration, can falsely identify legitimate emails as spoofed.
- Header Inspection Rules: Mimecast checks for many header requirements and if missing the check can trigger the Anti-Spoofing policies.
Key considerations
- Regular SPF review: Regularly check and update SPF records to include all valid sending sources, especially when using new services or vendors.
- DMARC alignment: Ensure DMARC settings are aligned with SPF and DKIM records, and understand the implications of 'reject' or 'quarantine' policies.
- Mimecast allowlisting: Use Mimecast's allowlisting feature to explicitly trust internal servers and authorized third-party senders.
- Log analysis: Regularly review Mimecast's message tracking logs to understand why emails are being rejected and adjust policies accordingly.
- EHLO validation: Specifically ensure that the SPF record passes validation on the EHLO domain.
- Proper Email Header Setup: Double check that the email headers meet the required standard and that no data is missing.
Marketer view
Email marketer from Quora shares that if you are using a third-party vendor to send email on your behalf, ensure that you give them the necessary permissions to send mail on behalf of your domain. Failing to do so, will result in the email being rejected if the mail platform is not listed as an allowed sender within the vendor's system.
30 Dec 2021 - Quora
Marketer view
Email marketer from Experts Exchange warns that Mimecast has header inspection rules that might consider a mail to be spoofed, even if SPF, DKIM and DMARC are properly set up. Make sure you inspect the headers to make sure that certain elements are not missing such as Reply-To.
15 Jul 2024 - Experts Exchange
What the experts say
3 expert opinions
Emails bounce due to Mimecast's Anti-Spoofing policies being triggered. Solutions involve creating an exception policy for the sender's address/IP, fine-tuning policy settings with Mimecast support, ensuring correct SPF/DKIM/DMARC configuration, utilizing Mimecast reporting, and maintaining up-to-date SPF records especially with third-party senders.
Key opinions
- Anti-Spoofing Policy Trigger: The bounce is directly caused by an Anti-Spoofing policy within Mimecast.
- Aggressive Policies: Mimecast's policies are known to be aggressive, potentially flagging legitimate emails.
- SPF Importance: Maintaining an up-to-date and accurate SPF record is critical, especially with third-party senders.
Key considerations
- Exception Policy: Create an Anti-Spoofing policy to take no action on the sender's address/IP to bypass the filter.
- Support Collaboration: Work closely with Mimecast support to fine-tune policy settings and troubleshoot issues.
- Authentication Accuracy: Ensure SPF, DKIM, and DMARC records are correctly configured to prevent false positives.
- Reporting Utilization: Utilize Mimecast's reporting tools to identify the cause of bounces and refine policy settings.
- Regular SPF Review: Regularly review and update the SPF record, particularly when using third-party senders, to include all authorized sending sources.
Expert view
Expert from Email Geeks shares the solution based on the bounce message link, stating that the message triggered an Anti-Spoofing policy. To resolve this, create an Anti-Spoofing policy to take no action for the sender's address or IP address.
8 Jun 2024 - Email Geeks
Expert view
Expert from Word to the Wise explains that Mimecast is known for having aggressive Anti-Spoofing policies that can sometimes cause legitimate emails to bounce. He suggests working closely with Mimecast support to fine-tune your policy settings and ensure that your SPF, DKIM, and DMARC records are properly configured to avoid false positives. He also suggests making use of Mimecast's reporting to help understand why a mail bounced in the first place.
18 Jan 2023 - Word to the Wise
What the documentation says
5 technical articles
Emails are bouncing due to Mimecast's Anti-Spoofing policies. Solutions involve configuring exceptions in Mimecast's Anti-Spoofing policy for legitimate senders based on sender IP/address/domain. Proper SPF records are crucial to list authorized senders and prevent flagging as spoofed. DMARC, built on SPF/DKIM, protects against unauthorized domain use; strict DMARC policies require perfect alignment. Validating email headers against standards is vital to prevent triggering filters.
Key findings
- Anti-Spoofing Configuration: Mimecast requires specific configuration to allow legitimate senders through Anti-Spoofing policies.
- SPF Importance: Accurate and comprehensive SPF records are essential for preventing emails from being flagged as spoofed.
- DMARC Impact: Strict DMARC policies can cause Mimecast to reject emails that fail SPF/DKIM alignment.
- Header Validation: Validating and adhering to email header standards can prevent triggering anti-spoofing filters.
Key considerations
- Exception Policies: Configure Anti-Spoofing policies in Mimecast to exempt legitimate senders based on their IP address, email address, or domain.
- SPF Record Accuracy: Ensure your SPF record includes all legitimate sending sources for your domain to prevent false positives.
- DMARC Policy Review: Understand the implications of your DMARC policy and ensure it aligns with your SPF and DKIM setup to avoid unintentional rejections.
- Header Compliance: Regularly validate email headers against established standards to prevent issues with Anti-Spoofing filters.
Technical article
Documentation from Microsoft Learn explains the importance of SPF records and how they prevent spoofing. Ensuring that your SPF record includes all legitimate sending sources for your domain is crucial. Any email sent from a server not listed in the SPF record may be flagged as spoofed.
1 Nov 2022 - Microsoft Learn
Technical article
Documentation from DMARC.org explains that DMARC (Domain-based Message Authentication, Reporting & Conformance) builds on SPF and DKIM to provide email domain owners with a way to protect their domain from unauthorized use, commonly known as email spoofing. If your DMARC policy is strict (p=reject), then Mimecast will reject those emails unless they perfectly align with SPF and DKIM standards.
17 Jul 2024 - DMARC.org
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