Mimecast Blocked Email? Why It Rejects You and How to Fix It

You send a campaign to a list of corporate contacts, and a slice of it comes back looking like this:

451 Internal resources are temporarily unavailable. Please try again later

Or the permanent version:

550 Administrative prohibition – envelope blocked

Importantly, both come from Mimecast, and the difference between them is the first thing to establish. One is a temporary deferral that delivers on its own if your mail server behaves correctly. The other never delivers until something changes. Senders routinely treat the first as the second, panic, and make delivery worse.

What Mimecast Is and Why It Blocked You

Mimecast is a secure email gateway. A company points its MX records at Mimecast instead of its own mail servers, so inbound mail hits Mimecast’s filtering infrastructure first and only surviving mail reaches the customer’s Microsoft 365 or Exchange tenant.

You can identify a Mimecast domain from its MX records: us-smtp-inbound-1.mimecast.com, eu-smtp-inbound-2.mimecast.com and similar regional variants — a region prefix, then smtp-inbound-, then a number. Some offshore regions use mimecast-offshore.com, for example je-smtp-inbound-2.mimecast-offshore.com.

Like Proofpoint, the other dominant B2B gateway, Mimecast is overwhelmingly a business filter — professional services, finance, healthcare, universities, local government. If you send B2B, or your consumer list contains work addresses, you send through Mimecast whether you planned to or not. Its blocks come from two separate places:

• Mimecast’s own reputation layer: global checks applied for all customers — IP reputation, greylisting, DNSBL lookups, spam and malware signatures. These hit you at every Mimecast tenant at once.
• The customer’s tenant policies: each admin configures Blocked Senders, Permitted Senders, anti-spoofing, content examination, and geographic restrictions. Mimecast enforces these but did not decide them.

This determines who can fix your problem. A reputation block is between you and Mimecast. A tenant policy block is between you and the recipient’s IT department, and no amount of contacting Mimecast will move it.

How to Confirm This Is Your Problem

Grep your logs for rejections from *.mimecast.com or *.mimecast-offshore.com, then run the check that matters most: separate your 4xx responses from your 5xx responses and count them. Mostly 4xx means you do not have a block — you have deferrals, and the question is whether you retry correctly. Mostly 5xx means a genuine rejection, and you need to know which layer issued it.

Then check whether failures span many recipient domains or cluster on one. Failures across dozens of unrelated Mimecast tenants point at the global reputation layer. Failures at a single company, while other Mimecast domains accept you fine, point at that company’s own policy.

Mimecast Error Strings, Decoded

Match your exact bounce string, then act on the last column.

Sourced from Mimecast’s SMTP error code reference and rejected and deferred messages guide.

Two footnotes on that table, because both trip people up.

The two 451 names are misleading, and the table above is correct. Recipient Temporarily Unavailable has nothing to do with the recipient — it means your sending IP was blacklisted after too many invalid connections. IP Temporarily Blacklisted is not a blacklisting at all — it means you reached your mail server’s connection limit. The names read as though they were swapped. They are not; this is Mimecast’s documented wording, and reading them literally sends you chasing the wrong fix.

421 Sender address blocked is the exception to the 4xx rule. Step 2 below tells you that a 4xx means the mail is still in play, and that is true of every other 4xx in the table. This one is a tenant Blocked Senders policy wearing a temporary reply code: it will never clear on retry, no matter how correct your retry schedule is. Treat it as a block and escalate to the recipient’s admin.

Greylisting: The Block That Is Not a Block

This is where most “Mimecast is blocking us” reports actually end.

Mimecast applies greylisting by default to messages from connections it has not seen before. It records a triplet — sending IP, envelope sender, envelope recipient — issues a 451, and waits. A real mail server retries; a spam cannon usually does not. When the same triplet returns inside the window, the message is accepted and remembered. The timing is specific: minimum wait 60 seconds, maximum window 12 hours, after which the message is logged as Sender Failed to Retry and is gone.

Two sender behaviors break this, both common in bulk infrastructure.

Retrying too fast. A queue that retries every 15 seconds never clears the 60-second floor. It burns its retry budget and reports a failure that was never a failure.

Retrying from a different IP. This is the one that catches ESPs. The IP is part of the triplet, so if the first attempt leaves from 203.0.113.10 and the retry leaves from 203.0.113.11 because your pool round-robins, Mimecast sees a new triplet and greylists again. A large rotating pool can defer the same message indefinitely while every individual IP looks healthy. Envelope sender rotation and VERP schemes that change the return-path per attempt fail identically.

Pin retries for a given message to the IP and envelope sender that made the original attempt. This one change fixes more Mimecast “blocks” than any allowlisting request.

Retry and backoff schedule for Mimecast destinations

Configure your MTA close to this for 4xx responses from Mimecast hosts. The specific intervals below are ours, not Mimecast’s — Mimecast does not publish a retry schedule. What it does publish are the two bounds this schedule is built to satisfy: a 60-second minimum wait before a retry counts, and a 12-hour maximum window before the message is discarded. Any schedule that respects both will work.

Same IP and envelope sender on every attempt. Hard stop at 12 hours rather than trickling past it. Mimecast permits up to 20 concurrent SMTP connections per IP; staying well below that ceiling is good practice during a reputation recovery. Escalate to a human if attempt six still returns 451.

The Most Common Reasons Mimecast Rejects Senders

1. Broken retry behavior

The most common cause by a distance. Confirm your retries clear 60 seconds and hold the same IP before investigating anything else.

2. Tenant Blocked Sender policies

An admin added your domain, address, or IP range to a Blocked Senders policy. Precedence matters: Blocked Sender policies override Permitted Sender policies, and a domain-level block beats an address-level permit. “Just allowlist us” may not work — their admin needs a specific Blocked Sender – Take No Action entry for your address, or must remove the domain block.

3. Poor IP reputation

550 Local CT IP Reputation - (reject) means Mimecast’s ongoing checks turned against your IP, usually after a run of 451 deferrals you ignored. Stale B2B lists are the typical root cause: corporate addresses decay fast as people change jobs, and abandoned mailboxes behind Mimecast gateways start hard-bouncing at scale.

4. Public RBL listings

Mimecast consults external DNSBLs. If your rejection names a specific blacklist, the fix is entirely outside Mimecast — use that blacklist’s own delisting process.

5. Authentication failures

SPF hard fails surface as 550 Administrative prohibition - envelope blocked, easily mistaken for a policy block. Check SPF alignment before emailing anyone’s IT department.

6. Individual user blocks

End users have a personal Managed Senders list via the digest email and Personal Portal. One click on “block sender” produces 550 Envelope blocked – User Entry for that person, and their admin cannot easily override it.

How to Fix It: Step by Step

Order matters, and the wrong order is the default instinct. Most senders open a remediation request first. Remediation is assessed on whether your behavior changed, so a request filed while the same traffic continues gets closed — and if the cause was a tenant policy or a retry bug, the request was never going to help. Work in this sequence: confirm → classify → fix your side → escalate to the right party.

Step 1: Confirm it is Mimecast

Look up MX records for the failing domains. If they do not end in mimecast.com or mimecast-offshore.com, you are looking at a different gateway with a superficially similar string.

Step 2: Classify deferral versus hard rejection

Split failures by response class. 4xx means the mail is still in play and the fix is your retry configuration. 5xx means the message is dead and you need to identify which layer killed it. Every later step differs by the answer. Our bounce-code reference explains 4xx versus 5xx across every receiver.

Step 3: Fix retry behavior first

For 4xx, audit your MTA against the schedule above: minimum interval over 60 seconds, retries pinned to the originating IP and envelope sender, and a window reaching past a few hours rather than giving up in twenty minutes. In a large share of cases the problem ends here, with no external request needed.

Step 4: Fix authentication and reputation

For 5xx reputation rejections, stop sending to Mimecast from the affected IP — pushing traffic through an active rejection reinforces the signal. Then:

• Verify SPF passes for your envelope sender domain, DKIM validates, and DMARC aligns
• Check the IP against public blacklists and delist directly at any that list you
• Run the affected list through verification and remove invalid addresses
• Suppress corporate contacts with no engagement in 90 days — Mimecast gives no feedback loop or reputation dashboard, so engagement is your only proxy signal
• Remove purchased or scraped data entirely

Step 5: Escalate to the correct party

For a tenant policy block (421 sender address blocked, 550 administrative prohibition with SPF passing, anti-spoofing, content examination, geographic restriction, user entry), your only route is the recipient organization. Reach someone there and ask them to pass a request to their email admin. Give them your sending domain, your sending IPs, the exact rejection string, and the timestamp of a failed attempt — their admin can find it in the Rejected and Deferred queue in seconds. Ask them specifically to check for a domain-level Blocked Sender entry, since that overrides any permit they add.

For a Mimecast-side reputation block, honesty is required about the current state. Mimecast historically ran a public sender feedback form at mimecast.com/senderfeedback for exactly this. That form has been withdrawn: the old URL now redirects to community.mimecast.com/s/sender-feedback/, which returns a 404, with no like-for-like public replacement published at the time of writing. Some Mimecast documentation still references it; the reference is stale.

To be precise about what does and does not exist: there is no sender-facing route. Mimecast’s DNSBL documentation is about Mimecast’s own outbound IPs and directs affected parties to Mimecast Support, which only its customers can use — it is not a path for a third-party bulk sender whose IP was blocked on inbound mail. Remediation therefore runs through the recipient’s admin, or through Mimecast Support for Mimecast-owned IPs.

What remains publicly is Mimecast’s community site and its support knowledge base. The more reliable lever is indirect: a Mimecast customer can raise a support ticket about a rejection affecting their inbound mail, and those tickets reach Mimecast’s messaging security team. If you know anyone at an affected recipient organization, asking them to open that ticket gets further than any channel available to you directly.

Step 6: Resume gradually

Once mail flows, treat Mimecast destinations as a fresh warm-up: a fraction of normal volume, only recently engaged recipients, low concurrency, and watch your 4xx rate as you climb. Rising deferrals mean stop.

How to Prevent Mimecast Blocks

Get retry logic right once

A 60-second-plus minimum interval, IP-pinned retries, and a multi-hour window. One configuration change removes an entire recurring class of failure.

Keep sending IPs stable

Greylisting rewards consistency. Rotating IPs unnecessarily, or moving a sending domain between pools, resets your standing with every Mimecast tenant at once.

Sunset corporate addresses aggressively

Business addresses decay faster than consumer ones and Mimecast gives you no complaint feedback to catch it. Suppress corporate contacts after 90 days of non-engagement, versus the 180 many senders apply to consumer lists.

Monitor deferrals, not just bounces

Track your 4xx rate to Mimecast hosts as a standing metric. Reputation degrades through deferrals before it becomes rejections, which gives you days of warning.

Ask for allowlisting at the start

For transactional and high-value mail to corporate customers, ask their IT team during onboarding to add your domain and IPs to Permitted Senders, which bypasses spam scanning, greylisting, and IP reputation checks. It does not bypass malware, attachment, or URL protection — those still apply.

Shared IP vs. Dedicated IP: Who Fixes What

On a shared IP pool, retry behavior, concurrency, and IP stability belong to your platform, and a competent one has already tuned them for gateways like Mimecast. Your responsibility is list quality — a neighbor’s stale B2B list degrades the pool’s reputation, and a good provider isolates that sender before it reaches you.

On a dedicated IP, the greylisting triplet, the reputation, and the retry configuration are all yours. Tenant policy blocks against your domain remain yours to resolve either way.

How Mailercloud Handles Mimecast Delivery

At Mailercloud we deliver over a billion emails a month, so gateway behavior is daily operational work rather than an occasional incident. Our infrastructure pins retries to the originating IP and envelope sender, respects greylisting intervals instead of hammering through them, holds concurrency down to security gateways, and surfaces deferral rates by receiving domain so a reputation slide is visible before it becomes a rejection.

If your corporate campaigns are deferring or bouncing at Mimecast domains, talk to our deliverability team — or start free with Mailercloud.

FAQ: Mimecast Blocked Email

Is a 451 from Mimecast a block?

No. It is a temporary deferral, most often greylisting. Retry after at least 60 seconds from the same IP and envelope sender and it will normally deliver on the second or third attempt. It only becomes permanent if you fail to retry within 12 hours, which Mimecast logs as “Sender Failed to Retry.”

How do I request removal from Mimecast as a sender?

Mimecast’s public sender feedback form has been withdrawn — the old URL now redirects to community.mimecast.com/s/sender-feedback/, which returns a 404 — with no direct public replacement at the time of writing. There is no sender-facing remediation route: Mimecast’s published DNSBL guidance covers Mimecast’s own outbound IPs and routes through Mimecast Support, which is a customer path, not one open to a third-party bulk sender whose IP was blocked inbound. For tenant policy blocks, the recipient’s admin is the only person who can remove the block. For reputation blocks, the most effective route is asking an affected Mimecast customer to raise a support ticket, which reaches Mimecast’s messaging security team.

Why do some Mimecast domains accept my mail while one rejects it?

You are hitting a per-tenant policy rather than the global layer. Each customer configures its own blocked senders, content rules, and geographic restrictions. Widespread failures indicate reputation or authentication; failures at one company indicate that company’s policy.

Does Mimecast have a postmaster dashboard or feedback loop?

No. Mimecast publishes no sender reputation dashboard and no feedback loop. Your bounce logs, deferral rates, and engagement data are the only visibility you get, which makes monitoring your 4xx rate more important than with providers that publish sender data.

Can I get around a Mimecast block by changing IP?

It works briefly and costs more than it saves. A new IP has no greylisting history, so every triplet gets deferred from scratch, and the behavior that caused the block follows you. If the block was a tenant policy against your domain, changing IP does nothing at all.

What does “550 Administrative prohibition – envelope blocked” mean?

It’s one of two things — a tenant Blocked Senders policy, or an SPF hard fail on your end — and they’re fixed by different people. Check SPF first: if your envelope-sender domain’s SPF passes, the block is the recipient’s own policy and only their admin can lift it. If SPF is failing, that’s yours to fix. The string alone doesn’t tell you which, so verify authentication before contacting anyone.

Why do my emails keep getting greylisted by Mimecast?

Almost always a retry problem. Mimecast greylists on a triplet of sending IP, envelope sender, and recipient — and two common bulk-sending habits break it: retrying faster than the 60-second minimum, or retrying from a different IP because your pool rotates. Pin each message’s retries to the IP and envelope sender that made the first attempt, wait at least 60 seconds, and keep trying inside the 12-hour window.