Why does email deliver to one recipient but get rejected for another with Mimecast?

Key findings

• Recipient-specific factors: Mimecast and similar security solutions often allow individual users to set personal spam thresholds or maintain their own allow and block lists. This means recipient 'X' could have more lenient settings or have explicitly whitelisted the sender, while recipient 'Y' has stricter rules or has taken actions (like marking previous emails as spam) that lead to rejection.
• Engagement history: Prior interaction with a recipient, such as replies or adding the sender to an address book, can positively influence future deliverability to that specific individual. Lack of such engagement with another recipient can result in different filtering outcomes.
• Domain-level reputation: While direct score boosts aren't typical for an individual email, a history of successful mail delivery to multiple users within the same domain can improve the sender's overall reputation with that domain's email security gateway. This positive reputation can sometimes aid delivery to new or less engaged recipients at the same domain.
• Spam score and content: The 554 error often indicates that a message exceeded a spam threshold or contained a detected 'signature' (potential virus or spam-like content). Even subtle differences in the email's content (or perhaps attachments that are processed differently for CC'd recipients) could trigger this for one but not another. Reviewing the content for common spam triggers is essential, as discussed in our guide on why emails go to spam.
• Mimecast's policy enforcement: Mimecast's security policies can be complex, incorporating various checks including SPF, DKIM, and DMARC authentication, content filtering, and reputation checks. A rejection with a 554 code specifically points to a security policy violation, often related to spam or virus detection. For more on specific Mimecast bounces, see our page on Mimecast anti-spoofing policy bounces.

Key considerations

• Manual actions by recipient: Recipient 'Y' might have manually quarantined or blocked the email from their Mimecast quarantine portal, or even marked it as spam, triggering a rejection. This is a common scenario when specific users don't wish to receive certain communications.
• Contacting the recipient: If you are not a Mimecast customer, the recommended first step is to contact the recipient (or their IT department) to have them adjust their Mimecast configuration to permit your address. This is often the quickest resolution for targeted rejections. Mimecast provides a Sender Feedback form for review requests.
• Analyze the bounce message: The 554 error message itself, along with any additional details (like a Mimecast reference ID), can provide clues. Mimecast's support documentation, such as their article on troubleshooting email delivery, can offer more context for specific error codes.
• Investigate sending reputation: While the immediate cause might be recipient-specific, persistent rejections can indicate broader issues with your sending IP or domain reputation. Regularly checking your sender score and ensuring compliance with best practices is crucial to avoid blocklists and ensure deliverability across various recipients.

Key opinions

• Individual recipient settings matter: Many marketers agree that individual recipient settings within Mimecast (or any similar system) can heavily influence delivery. This includes personal spam thresholds or whether a recipient has manually whitelisted or blacklisted a sender.
• Engagement as a factor: A key opinion is that engagement, like replies or adding to an address book, positively impacts delivery to that specific recipient. The absence of such engagement for another recipient makes them more susceptible to filtering.
• Domain reputation provides a halo effect: While not a direct score boost, successful mail flow to multiple recipients within the same domain can create a general positive reputation that indirectly aids deliverability to other users at that domain. This helps explain why one sales rep's email might be blocked when others aren't.
• Content and spam score: Marketers frequently point to email content or an internal spam score (not visible to senders) as the reason for rejection, especially with the 554 Mimecast error. This necessitates careful content review.
• Manual recipient actions: Another common perspective is that recipient 'Y' might have actively marked the email as spam or manually blocked it from their Mimecast quarantine, leading to the explicit rejection.

Key considerations

• Recipient dialogue is key: When a Mimecast rejection occurs for a specific recipient, marketers often advise reaching out to that recipient or their IT department. This direct communication can facilitate whitelisting or adjustment of internal settings.
• Mimecast feedback forms: For non-Mimecast customers, using Mimecast's sender feedback form is a practical step to request a review of rejections, as suggested by Mimecast's own documentation.
• Review email content: Even if content seems benign, a thorough review for spam triggers, excessive links, or suspicious attachments is prudent. The difference in delivery could be a subtle content flag. This is also key when looking into emails going to junk for a single internal recipient.
• Understand filter behavior: It's important to understand that mail filters do treat engaged recipients differently from non-engaged ones. This can lead to seemingly inconsistent delivery within the same organization.

Key opinions

• Complex filtering algorithms: Experts highlight that security gateways use complex algorithms that analyze multiple data points, not just content, including sender reputation, IP history, and authentication status (like SPF, DKIM, and DMARC). A subtle deviation in any of these can trigger a block for one recipient but not another. See our simple guide to DMARC, SPF, and DKIM.
• Recipient engagement feedback: Recipient engagement (or lack thereof) provides feedback loops to mail filters. If one recipient actively engages, it signals legitimacy, whereas non-engagement or explicit spam marking by another recipient significantly impacts sender reputation, potentially leading to immediate rejections.
• Dynamic reputation scores: Sender reputation is not static. It can fluctuate based on recent sending patterns, complaint rates, and engagement metrics. A good reputation with one part of an organization might not instantly transfer to all, especially if individual recipient actions are negative.
• Content variations: Experts agree that specific keywords, link structures, or even minor changes in formatting can subtly increase an email's spam score, pushing it over a threshold for one recipient while remaining below it for another.
• Proactive monitoring: Consistent monitoring of bounce codes and detailed delivery logs is essential for diagnosing these specific delivery issues. Generic rejection messages like the 554 Mimecast error require deeper investigation into reputation and content.

Key considerations

• Clean list hygiene: Maintaining a clean sending list and promptly removing inactive or bouncing addresses is critical to avoid triggering spam filters and blocklists at recipient gateways like Mimecast.
• Warm-up new sending patterns: If email content or sending volume changes significantly, it's advisable to warm up new sending patterns gradually to prevent security services from flagging them as suspicious.
• Understanding reputation nuances: Recognize that a sender's overall domain reputation, built over time through positive interactions, can create a 'halo effect' that generally improves deliverability for all recipients at that domain. However, individual negative actions can still override this. Learn more about understanding your domain reputation.
• Proactive blocklist monitoring: Even for seemingly isolated issues, regularly checking if your IP or domain has landed on any blocklists (or blacklists) is a good practice, as this can affect deliverability to various recipients and services. For a comprehensive overview, read our guide what happens when your domain is on a blacklist.

Key findings

• Mimecast 554 error cause: Mimecast's own documentation indicates that a 554 rejection signifies a detected signature, which could be a virus or a spam score exceeding the maximum threshold. The specific spam score, however, is not available to the sender or within the Mimecast Administration Console directly.
• Recipient configuration: Documentation suggests that if you are not a Mimecast customer and receive this error, you should contact the recipient to have them adjust their configuration and permit your address. This highlights the power of individual recipient settings within Mimecast.
• Personal allow lists: Mimecast allows recipients to maintain personal permit lists, which ensure future emails bypass hold queues. Conversely, a personal block list will result in direct rejection, explaining why one recipient gets the email and another doesn't.
• SPF and rejection: Technical forums suggest that emails originating from sources not explicitly defined in a sender's SPF record (especially with a strict policy like `-all`) will be rejected, making it harder for spammers. While SPF typically applies universally, nuances in recipient configuration or the specific email service can lead to varied enforcement.
• Individual email delivery: General email delivery principles confirm that when sending to multiple recipients, each message is handled individually. A delivery failure for one does not automatically imply failure for all, and separate bounce messages are expected.

Key considerations

• Utilize Mimecast feedback channels: Mimecast's official documentation provides a Sender Feedback form for external senders to submit review requests for rejected emails, offering a formal pathway to resolve issues. You can also refer to their troubleshooting email delivery guide.
• Verify recipient address: While not the primary cause in the described scenario, documentation notes that an incorrect recipient email address is a typical reason for a bounce. Always double-check recipient addresses, especially when dealing with specific rejections.
• Large attachments: Mimecast's troubleshooting tips also mention large attachments as a potential reason for bounce messages. If the problematic email included an attachment, its size or content type could be a factor in differential delivery.