Why does email deliver to one recipient but get rejected for another with Mimecast?
Matthew Whittaker
Co-founder & CTO, Suped
Published 9 Jul 2025
Updated 15 Aug 2025
7 min read
It can be perplexing when an email successfully reaches one recipient but is rejected for another, especially when both are within the same organization and protected by a robust email security gateway like Mimecast. I've seen this scenario play out frequently, and it often leads to head-scratching moments for senders and IT administrators alike. Understanding why this happens involves delving into the nuanced layers of how Mimecast processes incoming messages and how individual recipient configurations and behaviors can influence delivery.
Unlike a simple spam filter, Mimecast employs a sophisticated set of policies, threat intelligence feeds, and user-specific settings to determine whether an email is safe to deliver. This multi-layered approach means that even a seemingly identical email sent to two different people within the same company can encounter vastly different outcomes. The rejection isn't always a blanket judgment on the email or sender, but rather a specific decision influenced by a complex interplay of factors at the recipient's end.
The complexities of Mimecast's filtering mechanisms
Mimecast operates on a principle of layered security, assessing emails against a multitude of criteria before allowing them into an inbox. This isn't just about scanning for viruses or obvious spam, but also evaluating sender reputation, content analysis, and compliance with various configured policies. Each layer acts as a gate, and an email must pass through them all to be delivered.
These policies can be applied globally across an organization, to specific groups, or even down to individual users. This granular control is powerful for administrators but can create scenarios where one recipient receives an email while another does not. For instance, an organization might have a strict default policy, but certain departments or users might have relaxed settings to accommodate specific business needs.
The message "554 Email rejected due to security policies" is a common Mimecast rejection code. It indicates that the email was blocked because it triggered one of the recipient's configured security policies. This could be due to a detected virus, a high spam score, or a policy related to content, attachments, or links, as explained in their support documentation. You can learn more about rejected and deferred messages on Mimecast's Zendesk support pages.
Understanding Mimecast's layered security
Mimecast's filtering isn't a single switch; it's a sophisticated system with multiple evaluation points. This includes policies based on sender reputation, content analysis (e.g., spam scoring), attachment and URL protection, and even granular user-specific settings. Each email undergoes this comprehensive scrutiny, ensuring a thorough defense against diverse threats. The precise reason for rejection is often embedded in these layers.
User-specific policies and engagement history
One of the most common reasons for differential delivery is the influence of user-specific configurations and engagement history. Mimecast allows individual users to adjust their personal spam thresholds, whitelist trusted senders, or block unwanted ones. If Recipient X has whitelisted your domain or email address, or has frequently interacted with your emails (e.g., replying to them), Mimecast's filters will likely trust your messages more for them.
Conversely, Recipient Y might have stricter personal settings, or they may have previously marked your emails as spam, which would trigger Mimecast to apply more aggressive filtering for that specific user. Even if the broader organizational policy allows your email, an individual's actions or settings can override it for their mailbox, leading to a rejection or quarantine.
It's also worth noting that Mimecast's filters learn over time. Positive engagement (like replies or moving emails from quarantine) builds a sender's reputation with that specific recipient and their organization. Negative engagement (like spam reports) has the opposite effect. This dynamic reputation can result in seemingly inconsistent delivery within the same domain.
Recipient X: Positive engagement
Interactions: Recipient X has replied to previous emails, added your address to their contacts, or regularly opens and clicks your messages.
Whitelisting: Your email address or domain is explicitly whitelisted in their Mimecast personal settings.
Policy: Their individual spam threshold or security policies are less strict than default.
Recipient Y: Negative or no engagement
Interactions: Recipient Y has not interacted, or worse, has reported your emails as spam.
Blacklisting: Your email address or domain is manually blocked in their Mimecast settings.
Policy: Their personal spam threshold or security policies are stricter than default.
Content, sender reputation, and authentication
Beyond user settings, the content of your email and your overall sender reputation are critical factors. Mimecast aggressively filters out emails with suspicious content, which can include virus signatures, malicious links, or patterns that indicate spam. An email might pass for one recipient if it's part of an ongoing, trusted conversation, but be flagged for another if the content is deemed questionable in a fresh context or if it triggers a more general policy.
Your sender reputation, both IP and domain, plays a significant role. If your sending IP or domain is listed on a public blocklist (or blacklist) (like a Real-time Blockhole List or RBL), or has a history of sending problematic emails, Mimecast may be more inclined to reject your messages, especially to recipients who haven't explicitly whitelisted you. For a deeper understanding, review our guide on email blocklists.
Email authentication protocols like SPF, DKIM, and DMARC are fundamental to building and maintaining a good sender reputation. Mimecast heavily relies on these checks. If your email fails authentication (e.g., a DKIM body hash failure), it's more likely to be rejected. Mimecast has specific anti-spoofing policies that might block emails even if they pass SPF and DKIM but fail DMARC alignment, especially if the policy is set to 'reject'. Learn more about Mimecast anti-spoofing policies and how to resolve issues.
This table outlines common reasons for email rejections and how they can affect different recipients within a Mimecast environment:
Rejection factor
Impact on recipient X (good history)
Impact on recipient Y (no/bad history)
Spam score / content
Less likely to be rejected due to whitelisting or trust. May bypass some content filters.
Higher chance of rejection or quarantine if content is suspicious.
Sender reputation (IP/Domain)
Protected by individual trust settings, but severe reputation issues can still affect delivery.
Highly susceptible to rejections if listed on a blocklist (blacklist) or has poor reputation.
Email authentication (SPF, DKIM, DMARC)
May still deliver if failures are minor and recipient X's trust is high.
Likely to be rejected by strict DMARC or anti-spoofing policies.
Recipient's Mimecast settings
Individual whitelist/permit policies allow bypass of broader organizational blocks.
Stricter personal settings, or explicit block entries, lead to rejection.
When you receive a Mimecast rejection, the message often provides clues. For instance, a common rejection message might look like this:
Example Mimecast rejection messagetext
554 Email rejected due to security policies - [nv_1Qqk-OrSuvvG9nR619g.us139]
Troubleshooting selective Mimecast rejections
When faced with selective Mimecast rejections, the first step is to analyze the bounce message for specific error codes or explanations. If you have a contact at the recipient organization, especially within their IT department, reaching out is often the quickest path to resolution. They can check their Mimecast logs for your specific email and determine the exact policy that triggered the rejection.
If direct communication isn't an option, or if the issue persists across multiple recipients at the same organization, you can use Mimecast's Sender Feedback form to request a review of the rejected emails. Regularly verifying your SPF, DKIM, and DMARC configurations can also preempt many deliverability issues.
Actionable troubleshooting steps
Review bounce messages: Look for specific Mimecast error codes and rejection reasons to pinpoint the issue.
Contact recipient's IT: Request they check their Mimecast logs and policies for your sender.
Always ensure your SPF, DKIM, and DMARC records are correctly set up and aligned.
Regularly clean your email lists to remove inactive or invalid addresses.
Encourage recipients to whitelist your sending domain or email address.
Segment your audience and tailor content to minimize spam complaints and maximize engagement.
Common pitfalls
Ignoring bounce messages and not analyzing the specific rejection codes provided by Mimecast.
Failing to engage with recipient's IT when encountering persistent delivery issues.
Sending emails with generic or suspicious content that triggers Mimecast's spam filters.
Assuming blanket delivery based on success with one recipient at an organization.
Expert tips
Proactively test email deliverability to key Mimecast-protected domains.
Monitor your sender reputation using tools like Google Postmaster Tools.
Build consistent positive engagement with recipients to enhance trust signals for Mimecast.
For critical communications, consider alternative delivery methods if email consistently fails for specific recipients.
Marketer view
A marketer from Email Geeks says that when emails deliver to one recipient but not another with Mimecast, it's often due to individual recipient settings, such as personal spam thresholds or whether they have added the sender to their address book.
2020-04-22 - Email Geeks
Expert view
An expert from Email Geeks says that prior interactions, like replies from one recipient, can build a sender's trust profile with Mimecast, making subsequent emails to that recipient more likely to deliver successfully compared to others without such history.
2020-04-22 - Email Geeks
Navigating Mimecast's intricate filters
Dealing with Mimecast rejections, especially when delivery is inconsistent across recipients, highlights the complex nature of modern email security. It’s rarely a simple on/off switch; rather, it's a dynamic interplay of global policies, user-specific configurations, content analysis, and sender reputation signals. The fact that one recipient receives an email while another doesn't is a strong indicator that individual settings or prior interactions are at play.
To effectively navigate these challenges, I emphasize a proactive and investigative approach. This means not only understanding the specific Mimecast rejection messages but also maintaining excellent sender hygiene, ensuring proper email authentication, and being prepared to communicate with the recipient's IT or Mimecast support when needed. Consistent monitoring and adaptation of your sending practices are key to achieving reliable inbox placement, even in the face of advanced security gateways like Mimecast.
