How can I test email deliverability to mailboxes protected by Mimecast?

Key findings

• Mimecast reliance: Mimecast heavily relies on a combination of internal systems and external blocklists, such as Spamcop, for its filtering decisions.
• Customer-specific rules: Individual Mimecast customers can implement their own blocking rules for specific envelope senders, adding another layer of complexity to deliverability.
• Limited direct testing: There are no readily available public tools specifically designed to test deliverability or sender reputation exclusively for Mimecast-protected mailboxes.
• Logs as indicators: The primary method to determine if mail is being rejected by Mimecast is through analysis of your email logs.
• Spam folder placement: Discovering if an email is merely spam-foldered rather than outright blocked requires access to a Mimecast account or a specialized inbox testing service.
• Seedlist solutions: Some inbox testing services offer B2B seedlists that include Mimecast-protected addresses, which can provide insights into inbox placement.

Key considerations

• Authentication checks: Ensure your SPF, DKIM, and DMARC records are correctly configured. Mimecast heavily relies on these authentication methods for verification, as highlighted by EmailListValidation.
• Log analysis: Review your email sending logs for bounce codes or error messages specifically mentioning Mimecast, as these can indicate rejection issues.
• Direct engagement: The most effective way to understand deliverability to a specific Mimecast-protected mailbox is often to engage directly with the recipient or their IT department.
• Targeted testing: If you have access to a client's Mimecast-protected mailbox, send test emails and check their inbox and spam folders. This is crucial for troubleshooting specific business domains.

Key opinions

• Logs are primary: Most marketers agree that checking email logs is the initial and often only direct way to know if Mimecast is outright blocking your emails.
• Incomplete picture: While logs tell you if an email is rejected, they don't reveal if it's merely landed in the spam folder, which is a common concern for marketers.
• Seedlist utility: Several marketers look for inbox testing tools that offer B2B seedlists which specifically include Mimecast-protected addresses to get a better sense of inbox placement. For more information, read our guide on what services you can use for email deliverability.
• Direct client testing: Many marketers end up testing by sending emails to a client's Mimecast-protected mailbox and then relying on the client to report on the email's destination (inbox or spam).

Key considerations

• Engagement with recipients: Actively work with recipients using Mimecast to troubleshoot delivery issues, as they can check their internal Mimecast logs or quarantined messages. Consider contacting mailbox providers directly.
• Check public blacklists: Although Mimecast uses internal tools, ensure your sending IPs and domains are not listed on common public blocklists that Mimecast may reference (e.g., Spamcop). Our guide on what an email blacklist is provides more detail.
• Focus on reputation: Maintain a strong sender reputation through consistent sending practices and proper email authentication to increase your chances of successful delivery to Mimecast environments, as Mimecast emphasizes sender reputation.
• Consult Mimecast documentation: Familiarize yourself with Mimecast's official documentation on email security and filtering to better understand their approach, as discussed on Bridgeall's blog about Mimecast features.

Key opinions

• Internal tools and external lists: Experts confirm that Mimecast utilizes a combination of proprietary internal tools and a range of external blacklists (like Spamcop) for their blocking decisions.
• Customer control: It's noted that Mimecast allows its customers to implement custom block rules for specific envelope senders, which can impact deliverability on a per-client basis.
• Limitations of logs alone: While logs can indicate outright rejection, they don't provide visibility into whether an email lands in a spam folder, which requires deeper access or specialized tools.
• Need for direct engagement: The most reliable way to get a full explanation of deliverability to a Mimecast-protected mailbox often involves direct communication with the Mimecast team or the recipient's IT support.

Key considerations

• Continuous learning: The email deliverability landscape is constantly evolving, and Mimecast's filters adapt. Stay updated on best practices for avoiding spam filters.
• Reputation monitoring: Proactively monitor your sender reputation and ensure your domain and IP are not listed on any relevant public or private blocklists, as these impact Mimecast's perception of your mail.
• Authentication rigor: Pay close attention to your email authentication protocols (SPF, DKIM, DMARC), as Mimecast uses these to determine email legitimacy. This is crucial for avoiding issues like Mimecast anti-spoofing policy bounces.
• Consult industry guides: Keep an eye out for comprehensive troubleshooting guides being developed by industry experts (e.g., on Spam Resource) which can offer deeper insights into Mimecast deliverability.

Key findings

• Layered protection: Mimecast employs multiple inspection funnels for email scanning, cleansing, and isolating threats, moving beyond simple blocklists.
• Authentication emphasis: Official Mimecast guidance confirms a strong reliance on SPF, DKIM, and DMARC for verifying email legitimacy and preventing spoofing.
• Sender reputation impact: Mimecast's filtering decisions are heavily influenced by the sender's reputation, built over time through consistent, compliant sending behavior. Maintaining a good reputation is a practical guide to understanding your email domain reputation.
• Customer control over policies: Mimecast administrators have tools to customize inbound and outbound mail policies, including specific sender or domain blocklists, which can override global filtering.

Key considerations

• Policy configurations: Senders should be aware that their emails might be subject to specific policies configured by the recipient's Mimecast administrator, beyond general Mimecast rules. This includes checks against Mimecast SPF Check.
• Malware and spam detection: Documentation suggests Mimecast uses advanced threat detection, so emails with suspicious content, attachments, or links are at high risk of being blocked or quarantined.
• Direct delivery bypass prevention: Mimecast's setup often includes measures to prevent attackers from bypassing the gateway and delivering spam directly to Office 365 or other mail servers, as discussed by Practical 365.