How can I see why Mimecast blocked my email?

Accessing Mimecast's internal logs, often managed by the recipient's IT team, is the most direct way to understand why your emails are being blocked or quarantined. These logs provide specific reasons, such as policy violations or content flags. You can also use bounce messages to get initial clues.

Do standard email deliverability tools work for Mimecast?

No, most generic deliverability testing tools do not have direct Mimecast integrations or dedicated seedlist addresses for Mimecast. They primarily test against standard consumer mailbox providers. For Mimecast, you need specialized B2B seedlists or direct testing to a client's Mimecast-protected mailbox.

What is the most accurate way to test Mimecast deliverability?

The best way is to send to an actual Mimecast-protected mailbox that you have control over, or whose owner can share detailed delivery reports. This provides real-world data on how Mimecast handles your emails. Alternatively, some advanced seedlist services include Mimecast in their testing environments.

How can I improve my deliverability to Mimecast mailboxes?

First, ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Second, improve your sender reputation by sending relevant emails and maintaining a clean list. Third, ask the recipient's IT team to allowlist your sending IP or domain in their Mimecast account.

How can I test email deliverability to mailboxes protected by Mimecast? - Tools - Email deliverability - Knowledge base

Testing email deliverability can be complex, especially when your recipients use advanced email security gateways like Mimecast. These platforms are designed to protect mailboxes from spam, malware, and phishing attacks, but they can also inadvertently block legitimate emails if not properly configured or if your sender reputation isn't strong.

Unlike standard mailbox providers, Mimecast adds multiple layers of scrutiny, making it challenging to pinpoint exactly why an email might be delayed or blocked. This requires a targeted approach to testing and troubleshooting to ensure your messages reach their intended recipients.

Understanding Mimecast's role in email delivery

Mimecast functions as a cloud-based email security platform that sits in front of an organization's internal mail servers. It acts as a gatekeeper, processing all inbound and outbound email to apply various security policies, including spam filtering, malware detection, and data leak prevention. This means your emails first hit Mimecast's servers before ever reaching the recipient's inbox.

Their system analyzes emails based on a wide array of criteria, including sender reputation, content, attachments, and authentication protocols. It can quarantine suspicious emails, reject known spam, or apply specific policies based on the sender or recipient. Understanding this flow is crucial because it means a generic deliverability test might not fully reveal how Mimecast handles your mail.

In essence, Mimecast is an intermediary that adds a significant layer of filtering and policy enforcement. For successful deliverability, your emails must not only pass the recipient's mail server checks but also navigate Mimecast's comprehensive security policies.

The multi-layered approach

Mimecast employs various techniques to determine email legitimacy, going beyond basic spam filtering. This includes analyzing sender IP reputation, domain reputation, content scanning, and advanced threat protection measures like URL and attachment sandboxing.

While effective for security, these layers can sometimes be overzealous, especially for senders with fluctuating volumes or less-than-perfect email hygiene. Your deliverability depends on navigating these sophisticated filters effectively.

Core deliverability factors in a Mimecast environment

For any email to land in the inbox, fundamental deliverability factors must be in place. This includes proper email authentication (SPF, DKIM, DMARC), a solid sender reputation, and engaging content that doesn't trigger spam filters. When Mimecast is involved, these factors become even more critical because it rigorously enforces these standards.

A strong

Mimecast SPF record and correctly configured DKIM and DMARC policies are your first line of defense. Mimecast often has strict anti-spoofing policies that rely heavily on these authentication checks. If your email fails any of these, it's highly likely to be quarantined or rejected, even if your content is legitimate.

Beyond authentication, maintaining a high sender reputation is paramount. Mimecast, like other security gateways, uses various internal and external blocklists (or blacklists) and real-time threat intelligence. If your IP or domain appears on these lists, or if Mimecast detects suspicious activity, your emails will face significant delivery hurdles. Consistent, clean sending practices are key.

Authentication Protocol	Mimecast's role	Impact on deliverability
SPF	Verifies sender IP against published DNS records.	Prevents direct IP spoofing. Failure can lead to rejection or spam flagging.
DKIM	Checks email integrity using a cryptographic signature.	Ensures message wasn't tampered with. Failure increases spam score significantly.
DMARC	Provides policy for failed SPF/DKIM and reporting mechanism.	Critical for brand protection and anti-spoofing. Strict policies can lead to quarantine or rejection.

Testing strategies for Mimecast-protected mailboxes

Generic email deliverability testing tools may provide some insights, but they often lack the specific Mimecast-controlled mailboxes needed for a precise assessment. Many deliverability testing platforms offer seedlists that include common providers, but Mimecast's private nature makes direct testing harder.

The most effective way to test deliverability to Mimecast-protected mailboxes is to send emails to actual recipients within an organization that uses Mimecast. If you have a client or partner using Mimecast, ask if you can send test emails to a specific mailbox they control. This direct approach allows you to monitor the real-world behavior of your emails, from delivery to inbox placement (or quarantine/rejection).

When direct testing is not feasible, look for deliverability testing services that specifically mention Mimecast in their seedlist coverage. Some advanced tools maintain B2B seedlists that include various corporate email security gateways, Mimecast among them. This offers a simulated, yet effective, environment for assessing how your emails perform under Mimecast's scrutiny. You should also regularly monitor IP and domain blocklists to preemptively identify potential issues.

General deliverability testing

Scope: Focuses on common inbox providers like Gmail, Yahoo, and Outlook. Google provides detailed guidelines for bulk senders.
Insights: Provides a general inbox placement rate and identifies common issues like DMARC failures or content-based spam triggers.
Limitations: May not accurately reflect filtering by specific, complex security gateways. Can miss unique Mimecast policies.

Mimecast-specific testing

Approach: Requires sending to an actual Mimecast-protected mailbox or using a seedlist with Mimecast coverage.
Analysis: Focuses on Mimecast's specific logs and quarantine reports. Identifies how Mimecast's policies affect delivery.
Benefits: Provides precise data on Mimecast's filtering decisions, helping to fine-tune your sending strategy for these specific environments.

Troubleshooting common Mimecast delivery issues

If your emails are not reaching Mimecast-protected inboxes, the first step is to analyze bounce messages and, if possible, Mimecast's administrative logs. Bounce messages can often provide clues, such as SPF or DKIM failures, or an indication that the email was categorized as spam. Access to the recipient's Mimecast logs (via the recipient's IT team) can offer a definitive reason for rejection or quarantine.

One common issue is Mimecast's anti-spoofing policy. If your DMARC policy is not aligned or if it detects what it perceives as domain impersonation, it will block the email. Ensure your SPF and DKIM records are correctly configured and that your DMARC policy is set to at least p=quarantine or p=reject. In some cases, recipients may need to explicitly allowlist your sending IP or domain within their Mimecast account.

Another common issue is content-based filtering. If your emails contain suspicious links, unusual formatting, or keywords commonly associated with spam, Mimecast's content filters might flag them. Regularly review your email content, including subject lines and body text, to minimize potential triggers. You can also perform a quick email spam test to catch these before sending. Additionally, ensure your DNS records, including MX records, are correctly configured and pointing to Mimecast, as issues there can cause mail flow problems.

Confirming MX records with DNS lookupBASH

dig MX yourdomain.com

Final thoughts on Mimecast deliverability

Testing email deliverability to Mimecast-protected mailboxes requires a nuanced approach that goes beyond generic spam checks. Given Mimecast's sophisticated filtering capabilities, understanding their policies, ensuring strong email authentication, and maintaining a pristine sender reputation are paramount.

Direct testing through a client's Mimecast-protected mailbox or using specialized seedlist services are the most reliable methods. Coupled with diligent monitoring of bounce messages, DMARC reports, and open communication with your recipients' IT teams, you can effectively diagnose and resolve deliverability challenges with Mimecast.

Views from the trenches

Best practices

Ensure SPF, DKIM, and DMARC are fully configured and aligned for your sending domains. This is fundamental.

Maintain a clean sending list to minimize bounces and complaints that can negatively impact your reputation.

Regularly monitor DMARC reports for insights into your email authentication performance and potential issues.

Segment your audience and tailor content to reduce spam complaints and improve engagement metrics.

Establish open communication with recipients’ IT teams for easier troubleshooting and allowlisting.

Common pitfalls

Over-reliance on generic deliverability tools that do not account for Mimecast's specific filtering rules.

Ignoring DMARC reports, which can reveal authentication failures that Mimecast heavily penalizes.

Not maintaining proper sender reputation, leading to your IPs or domains appearing on blocklists.

Failing to review email content for spammy keywords, unusual formatting, or suspicious links.

Sending to outdated or inactive email addresses, increasing bounce rates and negatively impacting reputation.

Expert tips

Leverage the Mimecast Administration Console if you have access, as it provides detailed logs and delivery information.

Utilize

Mimecast's own SPF validator

to ensure your SPF records are correctly parsed.

Consider engaging with a deliverability consultant who has experience navigating complex enterprise gateways.

Expert view

Expert from Email Geeks says that for Mimecast, deliverability issues are mostly diagnosed by reviewing logs. If they are blocking you, their system will indicate why. They use a range of external lists for blocking, but their core tools are internal.

2024-01-02 - Email Geeks

Expert view

Expert from Email Geeks says that Mimecast also allows its customers to block specific envelope senders and manage their own filtering rules, which can lead to custom blockages.

2024-01-02 - Email Geeks