Why is Mimecast blocking emails, and how do you fix it fast?

If Mimecast has rejected a message, three checks resolve most cases: look at Message Center’s Rejected and Deferred queues to see the exact action and code, audit your organisation’s Blocked Senders and Allow/Block rules before blaming anything else, and if the Analysis tab flags SPF, DKIM, DMARC or IP reputation failures, fix those on the sending side and retry.

That order matters. Admins often jump straight to authentication fixes when the real cause is a manual block rule sitting quietly in the policy list. Others assume a policy is at fault when the Analysis tab clearly shows a failed DKIM check. Working through the console evidence first saves you from guessing.

• Check Message Center for the rejection or deferral code before touching anything else.
• Audit Blocked Senders and Allow/Block rules at organisation level; they override individual permit settings.
• If the Analysis tab shows authentication or reputation failures, fix the sender-side record or IP issue and retry delivery.

Pro Tip: Screenshot or export the rejected message entry before you change any policy. If the fix works, you’ll want that evidence to prove what was wrong in the first place.

Key Takeaways

Resolving a Mimecast block reliably comes down to reading the Analysis tab evidence before changing any policy, since the cause is almost always visible there.

Table of Contents

• How do you find rejected and deferred messages in Mimecast?
• What do Mimecast’s rejection and deferral codes actually mean?
• Where do Blocked Senders rules live, and what overrides them?
• Can end users release or permit their own blocked messages?
• How do senders fix authentication and reputation problems?
• What’s the step-by-step checklist to resolve a blocked delivery?
• How do you set policies that block less without opening the door?
• What do most Mimecast blocking cases actually come down to?
• How can Digistrat help if Mimecast keeps blocking your mail?

How do you find rejected and deferred messages in Mimecast?

Message Center is where every blocked delivery leaves a trail. Log in to the Administration Console, navigate to Monitoring > Message Center, and you’ll find separate queues for Rejected and Deferred messages, held items, and messages that were released or bounced. Filter by sender domain, recipient, or date range to narrow down a specific case, then click into an individual message to open its full record.

This is where the real diagnosis happens. Selecting an item exposes the Analysis tab, which shows the sequence of checks Mimecast runs against inbound mail: SPF, DKIM, DMARC alignment, RBL (Realtime Blackhole List) status, graymail scoring, and virus or spam signature detection. Each check returns a pass, fail, or skip, and the Message Tracking Analysis tab lays these out in order so you can see exactly which check triggered the rejection or hold.

Reading it properly takes three steps:

1. Open the message record and check the top-line action (Rejected, Deferred, Held, or Bounced) alongside the SMTP code.
2. Work down the Analysis tab checks in sequence. A DKIM fail followed by a DMARC fail usually points to a broken signing key or domain misalignment, not a Mimecast policy problem.
3. Note which check is marked as the deciding factor. Mimecast often runs several checks even after one has already failed, so the visible fail isn’t always the one that caused the block.

Once you’ve identified the cause, export the evidence. Message Center lets you pull records as XLSX or CSV, and you should also grab the full message headers and any non-delivery report (NDR) text the sender received. If you’re planning to hand this off to a sending domain’s admin, that export is your entire case file. Sending them a screenshot alone rarely gives them enough to act on; a full header dump with the Analysis tab result attached usually gets a same-day response.

What do Mimecast’s rejection and deferral codes actually mean?

The difference between a rejection and a deferral changes what happens next, and it’s worth understanding before you touch any settings. A 5xx code is a hard rejection: Mimecast has permanently refused the message, and the sending server won’t retry. A 4xx code is a soft deferral: the sending server is expected to retry later, and many legitimate messages do get through on a second or third attempt once whatever triggered the delay has cleared.

A Mimecast 554 rejection typically means one of two things: a security policy match, such as Anti-Spoofing Lockout, or a manual entry in the Blocked Senders list. Both show up clearly once you check the Rejected queue, which is why that’s the first stop rather than the last resort.

A few of these are worth a closer look:

• Anti-Spoofing Lockout fires when Mimecast detects a message that appears to spoof an internal domain; it’s a security feature, but it also catches legitimate forwarders and some CRM tools that send “from” an internal-looking address.
• RBL listings are often transient. Mimecast’s own guidance on rejected and deferred messages notes that IP-based blocks can clear on their own, but a pattern of repeated deferrals usually means the IP has a genuine reputation problem worth investigating.
• Sender Callback Verification failures happen when Mimecast tries to confirm the sending address is real and gets no useful response; this is common with poorly configured shared hosting.

If a message shows as permanently rejected with no NDR and no trace in the sender’s outbound logs, it’s usually gone for good. At that point, the fastest route is asking the original sender to re-transmit once the underlying cause is fixed, rather than trying to recover the original attempt.

Where do Blocked Senders rules live, and what overrides them?

Organisation-level blocks sit in Administration Console > Gateway > Policies > Allow/Block Rules, and they take precedence over almost everything an individual user can configure. This is the single most overlooked cause of “false positive” blocks: an admin spends an hour chasing an authentication issue that doesn’t exist, when a colleague added a domain-wide block rule six months ago and forgot about it.

Blocked Senders policies can match on envelope address or header address, and can be scoped to a specific sender, an entire domain, or a URL pattern found in message content. Envelope matching checks the actual SMTP path; header matching checks the visible “From” field. A block rule written for one but tested against the other is a classic source of confusion, because the message can pass one check and still fail the block rule.

Mimecast also applies a Policy Override hierarchy: a more specific rule generally takes precedence over a broader one, and organisation-level Block rules typically override a user’s own Permitted Senders setting. So a user adding a sender to their personal allow list won’t rescue a message that a domain-wide Block rule is actively rejecting.

Changing these rules safely follows a repeatable process:

1. Audit the existing rule list for anything matching the sender domain or IP before adding or removing anything.
2. Apply a temporary permit, or set the rule to “take no action” rather than deleting it outright, so you can restore it quickly if the change causes a new problem.
3. Send a test message and confirm delivery through Message Center rather than just trusting the sender’s word that it “went through.”
4. Log the change with a date, reason, and who made it.
5. Revert if the test fails or if it introduces an unrelated issue.

Pro Tip: Never delete a Block rule outright during troubleshooting. Disable it or scope it down first. If the block turns out to be unrelated to your case, you’ve just reopened a door someone closed for a reason.

Can end users release or permit their own blocked messages?

End users have real, if limited, power here. The Mimecast Outlook add-in and Personal Portal both offer Manage Senders, which lets a user release a message from On Hold and permit that sender going forward. This is genuinely useful for individual cases and takes the load off IT for routine misfires, according to Mimecast’s own guidance on managing senders.

Two settings matter here: Auto Allow and Permitted Sender (Auto). Auto Allow lets addresses a user has previously emailed bypass certain spam checks automatically, which cuts down on unnecessary holds but can also let a compromised or spoofed address through if the user isn’t paying attention.

• Users can release, block, or permit senders directly from On Hold notifications.
• Auto Allow speeds up delivery for known contacts but isn’t a substitute for proper authentication checks.
• None of this touches organisation-level Blocked Senders rules. If admin policy is rejecting the message outright, the user’s portal will show nothing to release, because the message never reached that stage.

How do senders fix authentication and reputation problems?

Most persistent Mimecast blocks trace back to something the sending side controls, not a Mimecast misconfiguration. If the Analysis tab shows SPF, DKIM, or DMARC failures, that’s your starting point, and it’s usually fixable within an hour once you know what to check.

• SPF: confirm the sending IP is listed in the domain’s SPF record and that the record hasn’t exceeded the 10-lookup limit, which silently breaks validation.
• DKIM: check the signing key is active and that the selector in the DNS record matches what the sending platform is actually using. A rotated key with an unpublished DNS record is a common, easy-to-miss cause.
• DMARC: alignment failures happen when SPF or DKIM pass individually but the domains don’t align with the visible “From” address. Mimecast’s Analysis tab will show this as a DMARC fail even when the individual SPF and DKIM checks pass.

IP reputation is the other major lever. If the same sending IP keeps showing up in deferrals, check whether it’s listed on a public RBL; Digistrat’s guide to checking an email blocklist walks through the lookup process and what delisting typically involves. Shared IPs on some ESPs (email service providers) carry another sender’s bad reputation, which is often the real story behind a “we’ve done nothing wrong” complaint.

Deferrals behave differently from rejections and deserve their own patience. A pattern of intermittent 4xx responses that eventually clear often signals a fluctuating IP reputation or a transient network timeout rather than a fixed block, and tracking that pattern across several attempts helps you decide whether delisting is worth pursuing or whether adjusting sending cadence solves it on its own, according to Mimecast’s own notes on rejected and deferred messages. Asking the recipient to temporarily Auto Allow your address for a test send is a legitimate short-term diagnostic step, though it shouldn’t become a permanent workaround for a reputation problem you haven’t actually fixed.

When you escalate to the recipient’s admin, send them everything at once: full message headers, the Message Center export if you have visibility into it, the exact NDR text, and precise timestamps. Vague reports (“our emails aren’t arriving”) get slow responses. Specific evidence gets same-day fixes.

Pro Tip: If you’re not sure your DNS records are set up correctly in the first place, don’t wait for a Mimecast block to find out. A quick review of your sending architecture and authentication setup will surface most SPF/DKIM/DMARC issues before a recipient’s filter does.

What’s the step-by-step checklist to resolve a blocked delivery?

Work through this in order, and most cases resolve within an hour.

1. Capture evidence. Get the NDR text, full headers, and timestamp from the sender.
2. Check Message Center. Find the exact rejection or deferral code and read the Analysis tab.
3. Audit Blocked Senders. Rule out an organisation-level block before assuming a technical fault.
4. Apply a temporary permit if no security risk is evident, to confirm the block is the actual cause.
5. Fix authentication or reputation issues if the Analysis tab points there, then retry delivery.
6. Log the outcome, including what was changed and whether it worked.

Admins own steps two, three, and six. Senders own step five when the fault is on their DNS or IP. Step one and four work best as a shared task, since the sender usually holds the NDR and the admin holds the console access. A reasonable SLA is same-day for the initial Message Center check, and 24 to 48 hours for a DNS or reputation fix to propagate and be retested.

How do you set policies that block less without opening the door?

Mimecast’s spam scanning settings offer relaxed, moderate, and aggressive sensitivity levels, each with different actions: tag the header, hold for review, or reject outright. According to Mimecast’s own spam scanning documentation, aggressive settings catch more genuine spam but also generate more false positives. Moderate is the sensible default for most organisations unless you’re under active attack.

• Use Auto Allow and Permitted Senders sparingly, and review the list quarterly rather than letting it grow unchecked.
• Log every manual policy change, including who made it and why.
• Set up scheduled Message Center exports so you can spot a rejection pattern before a client complains. Digistrat’s notes on deliverability monitoring cover what a sensible monitoring cadence actually looks like in practice.
• Keep a short incident playbook ready so a blocked delivery doesn’t turn into a half-day scramble every time it happens.

What do most Mimecast blocking cases actually come down to?

In practice, the majority of “Mimecast is blocking our emails” tickets trace back to one of two boring causes: an old Blocked Senders rule nobody remembered writing, or a DKIM key that got rotated on the sending platform without the DNS record being updated to match. Neither is exotic. Both are entirely preventable with a five-minute audit.

Digistrat’s deliverability reviews turn up the same handful of misconfigurations across UK and European clients repeatedly enough that it’s stopped being surprising. A shared-IP ESP with a neighbour’s bad reputation. A DMARC record set to p=reject before anyone checked alignment properly. A Block rule from a security incident two years ago that was never removed. A targeted audit usually finds the actual cause faster than trial-and-error policy changes, because it starts with the evidence in Message Center rather than a guess.

— James

How can Digistrat help if Mimecast keeps blocking your mail?

If you’ve worked through the checklist above and the blocks keep coming back, or you simply don’t have the time to chase down every DKIM selector and RBL listing yourself, that’s exactly the gap a focused audit closes. Digistrat’s email deliverability audit reviews your Message Center history, authentication records, and sending reputation together, then hands you a remediation plan with the specific fixes ranked by impact rather than a generic checklist.

The audit covers exactly the ground this article has walked through: pulling the rejection and deferral evidence, checking SPF, DKIM, and DMARC alignment, tracing IP reputation issues back to their source, and reviewing whether your own Blocked Senders and Allow/Block rules are helping or quietly working against you. For businesses that rely on email revenue, ongoing reputation monitoring catches the next problem before it turns into a support ticket. If you’d rather talk it through first, book a deliverability advisory session and bring your Message Center export.

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• How to Fix Email Deliverability Issues | Digistrat
• Email Deliverability Consultant UK | Digistrat
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• Email Blocklist - How to Check If Your Domain Is on one | Digistrat