Why are Sendgrid verification emails blocked by Mimecast when project invites are not?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 17 Aug 2025
6 min read
It can be incredibly frustrating when emails from the same sending platform, even the same domain, experience vastly different deliverability outcomes. You send a project invite through SendGrid, and it lands perfectly. Then, a verification email, also via SendGrid (though triggered by Auth0), gets blocked by Mimecast. The error message, "550 Envelope blocked - User Entry," suggests a user-specific block, but if both are from the same domain, what gives? Let's unravel this common deliverability puzzle.
Understanding Mimecast's advanced filtering
Mimecast functions as a comprehensive email security gateway, designed to protect organizations from threats like spam, malware, and phishing. It doesn't simply look at the visible 'From' address. Instead, it employs a sophisticated array of filtering mechanisms that analyze numerous email attributes, including content, sender reputation, and various policy configurations.
This means that even if two emails originate from the same domain and IP address, Mimecast can distinguish between them based on subtle differences. These distinctions can trigger different policy responses. It's not always about a blanket block or whitelist for an entire domain; often, the rules are much more granular, targeting specific sender addresses, subject lines, or even patterns within the email body.
The error "550 Envelope blocked - User Entry" is particularly telling. It indicates that the block isn't necessarily due to a system-wide reputation issue with your SendGrid IPs or domain. Instead, it points to a specific policy configured by the recipient's organization or by the end-user themselves. This often means a targeted blocklist entry (or blacklist entry) for that particular sender or email type.
Differences in email characteristics
While you confirmed both emails originate from Auth0 via SendGrid, the crucial detail lies in the specific sender identities used. The RFC 5321.From (or envelope sender) for both might be similar, like bounces+<ID>@em5118.clearstory.build, but the RFC 5322.From (header from) address is different: noreply@ for invites and support@ for verification. Mimecast policies can specifically target these header from addresses.
Beyond sender addresses, the content and purpose of the emails differ significantly. Project invites are typically less frequent and serve as initial engagement. Verification emails, on the other hand, are critical transactional messages, often containing unique links or codes, and are typically part of an authentication flow. Security gateways like Mimecast might apply stricter scrutiny to emails that could potentially be used for account takeover or credential stuffing if compromised.
Even with correct SPF, DKIM, and DMARC authentication passing (which you should always verify), the perceived nature of the email can influence Mimecast's decision. If an organization frequently receives unsolicited or malicious emails disguised as support@ emails, or if the support@ address has previously been associated with spam (even innocently), it could be on an internal blocklist.
Project invite email
Sender address: Usually noreply@yourdomain.com. Often perceived as less critical and more marketing or notification oriented.
Content: Contains an invitation or general information. May have fewer elements that trigger strict security policies.
Frequency: Sent less frequently, on an ad-hoc basis as projects are created.
Recipient expectation: Users are actively expecting these invites, making them less likely to be marked as spam.
Verification email
Sender address: Often support@yourdomain.com or security@yourdomain.com. These addresses are frequently spoofed by phishers, making them high-risk.
Content: Contains sensitive links for account activation or password resets. These are prime targets for malicious filtering.
Frequency: Transactional and potentially high volume depending on signup rates.
Recipient expectation: While expected, the transactional nature can still trigger aggressive filtering if anything seems off.
Mimecast policies and user overrides
Mimecast's policy engine is highly customizable, allowing administrators to define specific rules for inbound and outbound messages. These rules can be based on a myriad of factors, including sender address, subject line keywords, attachments, URL categories, and even a blocked senders list. The "User Entry" message strongly suggests that the blocking is a direct result of such a policy, rather than a general spam classification.
It's possible that the recipient's IT team or even the individual recipient themselves has explicitly added support@yourdomain.com to a personal or company-wide blocklist (or blacklist) within Mimecast. This might happen if previous verification emails were perceived as too frequent, confusing, or simply unwanted for certain users. For example, if a user tried to sign up multiple times and generated many verification emails, they might block them to stop the noise.
Policy-based blocking
Mimecast allows highly specific policy configurations. A policy could be set to block emails from support@yourdomain.com if it contains certain keywords (like 'verify' or 'activation') or links of a specific type. This provides a level of control beyond simple domain blocking.
Additionally, Mimecast may utilize reputation scores for specific sender addresses or even patterns. If the content or sending behavior of verification emails deviates from what Mimecast expects for a good sender, it could be flagged, leading to a block.
Investigating and resolving the issue
To effectively troubleshoot this, your primary goal is to gather more specific details from the recipient's Mimecast logs. Since the error explicitly mentions "User Entry," it's highly likely that a direct rule or blocklist (blacklist) exists for the support@ address or for verification emails in general.
First, ensure your authentication records are fully compliant and aligned. Use a blocklist checker to check if your sending IP or domain is listed. While the "User Entry" implies a specific block, a general blacklist status can exacerbate any existing issues. Review your DMARC reports for any anomalies related to your support@ sender. You might find valuable insights into how these emails are being treated differently at the authentication level. If you are having issues with authentication, checking your DMARC monitoring is important.
The most direct path to resolution is to contact the recipient and request that they whitelist your specific sending address, support@yourdomain.com. Their IT department should be able to check their Mimecast logs for the exact reason the email was held and adjust their policies accordingly. Since it's a user entry, it means the control is on their side. You might also consider using a distinct subdomain for verification emails, like verify.yourdomain.com, to separate its reputation and avoid blanket blocks on your main domain.
Conceptual Mimecast blocked senders policyText
Mimecast Blocked Senders Policy Example:
Policy Name: Block Support Email Verifications
Applies To: Inbound Email
From: support@yourdomain.com
To: specific_recipient@targetdomain.com (or wildcard)
Action: Block
Notes: User-requested block due to excessive verification emails.
Views from the trenches
Best practices
Maintain separate sending subdomains for different email types, such as marketing, transactional, and verification.
Regularly monitor DMARC reports to identify authentication failures and gain visibility into how your emails are being handled by receiving servers.
Segment your email lists and send only to engaged recipients to maintain a healthy sender reputation and avoid spam complaints.
Keep email content clean, relevant, and free of elements commonly associated with spam, even in transactional emails.
Common pitfalls
Using a single sender address like 'support@' for all types of communications, which can lead to blanket blocking if one type triggers a filter.
Failing to review detailed Mimecast logs or engage with the recipient's IT team to understand specific block reasons.
Ignoring the distinct reputation profiles that different email streams can develop, even when sent from the same platform.
Not having a clear unblock request process for recipients or their IT departments to follow when issues arise.
Expert tips
If possible, get a copy of the bounce message and the full email headers for both the delivered and blocked emails. These can reveal subtle differences in the mail flow or attributes that Mimecast is reacting to.
Consider asking the recipient to check their Mimecast 'Managed Senders' list or other individual policies. The 'User Entry' block strongly points to a direct user action.
Ensure your SendGrid domain authentication (SPF, DKIM, and DMARC) is fully set up and passing for all sending domains and subdomains. Misconfigurations can sometimes cause intermittent blocking.
Test your transactional email deliverability rigorously, as these are often more sensitive to filtering due to their immediate impact and potential for abuse.
Expert view
Expert from Email Geeks says that understanding the specific RFC 5321.From and RFC 5322.From domains is crucial. Even if the envelope sender is the same, Mimecast may filter based on the header 'From' address, especially if it's a commonly spoofed address like 'support@'.
2024-02-08 - Email Geeks
Marketer view
Marketer from Email Geeks says that the '550 Envelope blocked - User Entry' error is often an indication that the recipient or their IT department has manually added the sender to a blocked list in Mimecast. It's not usually a system-wide reputation issue.
2024-02-09 - Email Geeks
Navigating granular email security
The core of the issue often lies in the granular filtering capabilities of email security gateways like Mimecast. While both your project invites and verification emails come from SendGrid, the subtle differences in sender addresses (like noreply@ vs. support@) and content can trigger distinct policies, especially when a user-specific block is in place. To address this, it's essential to investigate the specific Mimecast error details, collaborate with the recipient's IT team, and ensure your email authentication and sending practices are robust for all email types.
Understanding why your transactional emails are blocked can be a complex task, especially when other emails from the same service deliver successfully. Often, the solution lies in a detailed analysis of your email headers and a clear understanding of the recipient's email security policies, which might include specific per-recipient filtering rules.
SendGrid, and it lands perfectly. Then, a verification email, also via SendGrid (though triggered by Auth0), gets blocked by 
Mimecast. The error message, "550 Envelope blocked - User Entry," suggests a user-specific block, but if both are from the same domain, what gives? Let's unravel this common deliverability puzzle.
Auth0 via SendGrid, the crucial detail lies in the specific sender identities used. The RFC 5321.From (or envelope sender) for both might be similar, like bounces+<ID>@em5118.clearstory.build, but the RFC 5322.From (header from) address is different: noreply@ for invites and support@ for verification. Mimecast policies can specifically target these header from addresses.
