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Provider deliverability · Mimecast (secure email gateway)

Why is Mimecast blocking my emails?

Samuel Chenard

By Samuel Chenard · CEO & Co-Founder, Palisade · Reviewed September 19, 2026

Mimecast blocks mail at the recipient's gateway, not at yours, so the rejection is the receiving organization's policy rather than a verdict on your mailbox. The usual causes are their Anti-Spoofing policy, a Blocked Senders policy, an SPF hard fail their DNS Authentication policy rejects, and poor sending IP reputation. Mimecast's bounce text names the policy.

The 30-second check

Mimecast returns a plain-language reason with every rejection, and two of the most common reasons come straight from your SPF record: an SPF hard fail is rejected outright under Mimecast's recommended inbound settings, and the anti-spoofing bypass a recipient's administrator would create for you is scoped by sending IP. Check what your SPF record actually authorizes before you ask anyone for an exception.

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Why Mimecast is blocking your email

Likely causeWhat's happening
Their Anti-Spoofing policy caught you sending as their domainMimecast describes the purpose plainly: "Anti-Spoofing policies ensure that external messages appearing to come from an internal domain are blocked." If you send on behalf of the recipient's own domain from a marketing platform, a CRM, or an application server, your mail arrives from outside their gateway carrying their domain in the From address, which is exactly the pattern the policy exists to stop. The bounce reads "Anti-Spoofing policy - Inbound not allowed" or "Rejected by header-based Anti-Spoofing policy".
Your sending IP is not in your SPF recordMimecast's recommended inbound DNS Authentication settings reject on SPF Hard Fail, with the note that "in these cases, the sender is not sending the message from an authorized IP address." The bounce is "SPF Sender Invalid - envelope rejected". Mimecast also documents that when several checks disagree, "the most restrictive action wins", so an SPF hard fail alone is enough to end the delivery.
A Blocked Senders policy or a Managed Senders entry names youMimecast has three separate layers that can list you: an administrator's Blocked Senders policy, a profile group holding a whole domain, and an individual recipient's own Managed Senders list. Mimecast states that "Blocked Sender policies are designed to take precedence over Permitted Sender policies", so being on somebody's permitted list does not save you. The bounce reads "Administrative prohibition envelope blocked", "Envelope blocked – User Entry", or "Rejected by header-based Blocked Senders - Block policy for Header From".
Greylisting, or a reputation check that has not cleared yetMimecast greylists unfamiliar sources with a 451 and the reason "Internal resources are temporarily unavailable." Its documentation says this "requires the server to retry the connection between one minute and 12 hours", and that the same code also appears when "the sender's IP address has a poor reputation". A sending server that does not retry inside that window never delivers at all.
Ongoing IP reputation checks turned the deferral into a rejectionMimecast documents a separate permanent failure for reputation: "Local CT IP Reputation - (reject)", described as "ongoing reputation checks have resulted in the message being rejected due to poor IP reputation. This could occur after a 4xx error." That last sentence is the sequence to watch for. Deferrals that you ignore harden into rejections.
Your IP is on a blocklist Mimecast consultsMimecast returns a 550 carrying "< details of RBL >", and says "the text displayed is specific to the RBL, which lists the sender's IP address." Its own recommended remedy is for the recipient to bypass the listing with an Auto Allow or Permitted Senders policy and for you to "request the associated IP address from the RBL", which means getting delisted at the source.
A Content Examination policy matched something in the messageMimecast rejects with "Message bounced due to Content Examination Policy" when the recipient's content rules match. Mimecast notes that "multiple content examination policies can conflict and potentially remove legitimate emails, especially when policies are broadly applied without specific group targeting." Nothing on your side fixes this one. It is a rule in their tenant.
You opened too many connections or listed too many recipientsMimecast defaults to 20 concurrent inbound connections per account and returns "Account service is temporarily unavailable" past that, with the IP automatically removed from the block list after five minutes. It also accepts 100 RCPT TO entries per message body by default and returns a 452 "Too many recipients" as a truncation request beyond that.

Check the public signals before changing settings

The free check above gives you a fast public baseline for how Mimecast sees your mail. It does not replace the provider's private reputation or placement data, but it tells you whether the problem is visible from outside before you edit a sending platform.

Palisade SPF checker result showing a published SPF record, authorized mechanisms, and lookup analysis for a non-sensitive test domain.
Source: Palisade, “SPF checker”, checked 2026-07-29. First-party public tool result for a non-sensitive test domain; it evaluates public SPF DNS only.

How to fix it, step by step

  1. Check what your SPF record authorizes

    Use the free checker above, or open the SPF checker page directly. Mimecast rejects an SPF hard fail outright under its own recommended inbound settings, so a sending service missing from your record is a rejection you can fix without contacting anybody.

  2. Read the reason string in the bounce, not just the number

    Mimecast publishes its 4xx and 5xx reason strings with a cause for each, and the string is the diagnosis. "Anti-Spoofing policy - Inbound not allowed", "SPF Sender Invalid - envelope rejected", and "Envelope blocked – User Entry" are three completely different problems behind one 550.

  3. If it names Anti-Spoofing, stop presenting as their domain

    Either send from your own domain, or ask the recipient's Mimecast administrator to add a Take No Action anti-spoofing policy scoped to your sending IP range. Mimecast's own guidance for that bypass is to tighten it by entering "the sending server's public IP address or address range in CIDR format", so have that range ready.

  4. If it names Blocked Senders, ask their administrator to permit you

    Mimecast's documented path is Email Delivery | Managed Senders in the Administration Console, where an administrator searches for the sender and selects Delete or Permit Sender. For a domain-level block, Mimecast says a Take No Action policy has to be created for your specific address, because the domain block otherwise still applies.

  5. Confirm your own server retries a 451 properly

    Greylisting only works if your MTA comes back. Mimecast expects a retry between one minute and 12 hours; miss the window and the attempt is logged as "Sender Failed to Retry". Check your queue settings before assuming you were blocked.

  6. Clear blocklist listings on your sending IPs

    Run each sending IP through the free blocklist checker and delist at the source. Mimecast's RBL rejection carries the listing service's own text, so the bounce usually tells you which list to work on.

  7. Publish DMARC and take it to enforcement

    Anti-spoofing is the gateway's guess at whether mail claiming your domain is really yours. A DMARC record at p=reject, with SPF and DKIM correct for every legitimate sender, turns that guess into a published instruction. Check the record at the DMARC checker.

Related free tools: DMARC checker · Blocklist checker · IP reputation · Email header analyzer

If you send in volume: what Mimecast publishes instead of sender rules

Mimecast is a secure email gateway sitting in front of a customer's mailboxes, not a mailbox provider, and it publishes no bulk-sender requirements the way Google, Yahoo and Microsoft do. The limits it does publish are per-connection and per-account rather than per-day: 20 concurrent inbound connections per customer account by default, after which the IP is temporarily blocked and automatically released five minutes later, and 100 RCPT TO entries per message body, after which Mimecast returns a 452 that most sending servers treat as a request to truncate and continue. Neither is a reputation judgement. The requirements that decide whether your mail reaches a Mimecast customer are that customer's own policies, plus the mailbox provider behind the gateway. Per Mimecast's SMTP error code documentation, checked 2026-09-02.

Bounce codes you may be seeing

A block from Mimecast surfaces as a specific SMTP code. Some lists below also carry codes returned by the systems receiving your mail; each line names its issuer. Match yours below; where a code has a full guide, it is linked.

The real root cause: unenforced authentication

Two of Mimecast's most common rejections are the same question asked twice: can this message be tied to the domain it claims? The anti-spoofing policy asks it about the header From address and blocks external mail wearing an internal domain. The DNS Authentication policy asks it about the envelope and rejects an SPF hard fail. Both are decided before a human sees anything, by records you publish and they read. That is why chasing exceptions is the slow path. Every bypass a recipient's administrator creates for you is scoped to an IP range, has to be maintained, and only helps at that one company. Complete SPF for every service that sends as your domain, DKIM signing with your own domain, and a DMARC record walked from p=none up to p=reject make the answer unambiguous at every gateway at once, Mimecast included. Aggregate reports are how you find the senders you forgot; enforcement is what stops the ambiguity coming back.

DMARC software that does the work

Palisade's AI agent takes domains all the way to enforcement: hosted SPF, DKIM, DMARC, and MTA-STS records on paid plans, DMARC reports monitored continuously, and every policy step drafted for your approval on the way to p=reject. The full product is open for a 15-day trial, and nothing is charged until it ends.

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Fixing this across every client domain

Mimecast blocks land on your desk as a client ticket that is not actually about your client's tenant. It is about a recipient's gateway, a bounce string nobody has read carefully, and an SPF record that never listed the CRM. Palisade gives you the fleet view instead of the one-off: hosted and managed SPF, DKIM, DMARC and MTA-STS records for every client domain, aggregate reports read for you so every sending service is named rather than guessed, and a path to p=reject with your team approving each change. Native ConnectWise, HaloPSA and Autotask integrations put it in your PSA, pricing is per client domain with rates that improve as the portfolio grows, and your own MSP domain is a free NFR domain to run the process on before any client conversation.

Questions readers ask

Frequently asked questions

What does "550 Rejected by header-based Anti-Spoofing policy" mean?

That rejection means the recipient's Mimecast gateway saw an external message carrying one of their own domains in the header From address. Mimecast blocks that pattern by design. It is common when a third-party platform sends on behalf of a customer's domain. The fix is a Take No Action anti-spoofing policy scoped to your sending IP, created by their administrator.

How do I get whitelisted in Mimecast?

Only the receiving organization can whitelist you in Mimecast, because the policies live in their tenant. Their administrator adds your address to a Permitted Senders profile group, or creates a Take No Action policy. Mimecast notes that a Permitted Sender entry does not survive an anti-spoofing hit, so the exception has to match the policy that actually rejected you.

Why is Mimecast still blocking me when I am on the permitted senders list?

Mimecast ranks its policies, and permitted senders do not sit at the top. Its documentation states that Blocked Sender policies take precedence over Permitted Sender policies, and separately that anti-spoofing policies override addresses permitted by users. Being permitted clears spam and reputation checks, not a domain-level block or a spoofing verdict.

What is Mimecast greylisting and how long does it last?

Mimecast greylisting is a deliberate temporary failure on unfamiliar senders, returned as a 451 with the reason "Internal resources are temporarily unavailable". Mimecast documents that the sending server must retry between one minute and 12 hours. A correctly configured server retries and delivers. A server that gives up logs as "Sender Failed to Retry" and the mail is lost.

Does Mimecast reject email on an SPF hard fail?

Mimecast rejects on SPF hard fail under the inbound configuration it recommends to its own customers, returning "SPF Sender Invalid - envelope rejected". Softer results are handled differently: none, neutral, softfail, permerror and temperror are all recommended for spam and reputation scanning rather than rejection. Each customer can change these settings, so behaviour varies by tenant.

Does Mimecast honour my DMARC record?

Mimecast can honour your DMARC record, and that is its recommended inbound setting. For a DMARC Fail result Mimecast recommends the Honor DMARC Record action, which applies the policy you published. That cuts both ways: if you publish p=reject while a legitimate sender is unauthenticated, a Mimecast customer will reject that mail because you told them to.

Can I contact Mimecast directly to get my IP unblocked?

Mimecast does not offer a public sender delisting portal, and its error-code documentation says that if you hit these errors sending to a Mimecast customer you should contact the recipient's Mimecast administrator, because "Mimecast can only deal with designated customer contacts". Its documentation does point to an online form for requesting a review of source IP ranges.

Why did Mimecast block my email when the recipient says they want it?

Mimecast decisions are made by gateway policy before any mailbox is involved, so a recipient wanting your mail changes nothing on its own. Worse, a recipient can block you themselves: Mimecast documents user-level Managed Senders entries and a Block All option in digest notifications, both of which produce rejections their administrator has to undo.

Sources and last verified

Every claim about Mimecast on this page links to the source it came from, each with the date it was last checked. Provider policies change; if a detail looks off, the linked source is authoritative.

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