Why are emails bouncing with Mimecast Anti-Spoofing policy and how to fix?

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.

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.

Email marketer from SuperUser explains that incorrect DMARC settings can lead to Mimecast falsely identifying emails as spoofed. If your DMARC policy is set to 'reject' or 'quarantine', any email that fails SPF and DKIM checks will be blocked. Ensure your DMARC record is properly configured and aligned with your SPF and DKIM records.

Email marketer from StackExchange shares that Mimecast can sometimes block emails from subdomains if it's not configured to recognize them as part of the main domain's trusted sources. Ensure that your SPF and DKIM records include the subdomain, and that Mimecast is configured to recognize the subdomain as a valid sending source.

Email marketer from Email Geeks suggests checking if the SPF record passes on the EHLO domain, as Mimecast might not perform the SPF check on the return-path domain.

Email marketer from Reddit shares that Mimecast often blocks internal emails if the sending domain is not properly authenticated. Ensuring SPF, DKIM, and DMARC are correctly configured for your domain can help resolve these bounces. Also, adding the internal mail server's IP address to the Mimecast allowed senders list can prevent false positives.

Email marketer from Spiceworks explains that a common fix for Mimecast Anti-Spoofing issues is to add the sending server's IP address or domain to the Mimecast allow list. This tells Mimecast to trust emails originating from that source, preventing them from being incorrectly flagged as spoofed. You can find this setting under 'Sender Policies' in the Mimecast admin console.

Email marketer from MXToolbox says to make sure that if internal servers are sending mail, Mimecast is aware of the valid IP addresses to prevent this exact issue. Internal relays need to be identified to Mimecast as 'trusted' and/or SPF records need to contain all valid relay IP addresses.

Email marketer from Email Geeks explains that Mimecast might block emails from similar domains or subdomains handled externally due to enabled Anti-Spoofing. The solution is to allowlist the external source.

Email marketer from Mimecast Support Forums recommends reviewing Mimecast's message tracking logs to identify the specific Anti-Spoofing policy that's triggering the bounces. The logs will provide details on why the email was rejected and which policy was triggered, allowing you to adjust the policy accordingly.

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.

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.

Expert from Word to the Wise stresses the importance of maintaining an up-to-date SPF record, particularly when using third-party senders. An SPF record that includes outdated or incorrect IP addresses can lead to authentication failures and cause emails to be flagged as spoofed. She advises regularly reviewing your SPF record and ensuring that it accurately reflects all authorized sending sources.

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.

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.

Documentation from Mimecast describes that Anti-Spoofing policies consist of conditions (such as sender IP, sender email address, or domain) and actions (like rejecting, quarantining, or allowing the message). The policy should be configured to exempt legitimate senders from being flagged as spoofed.

Documentation from Mimecast Community explains that to resolve bounces due to Anti-Spoofing policies, you need to configure an Anti-Spoofing policy to take no action for the sender's address or IP address. This is done through the Mimecast Administration Console by creating or modifying an existing policy under Gateway | Policies | Anti-Spoofing.

Documentation from RFC-Editor explains the importance of validating email headers. It helps provide a standard on making sure that the headers meet the correct specifications. Some email platforms will flag missing or incorrect headers, thus increasing the likeliness of an Anti-Spoof filter from triggering.