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When MailChannels Inbound Filtering cannot process a message as expected, it returns an SMTP error response to the sending mail server. This page explains what each error means and what to do about it. If you believe a legitimate message was incorrectly rejected, see Report misclassified email.
This response indicates that the message has been greylisted. Greylisting is a spam-reduction technique where MailChannels temporarily defers mail from an unrecognized sender, requiring the sending server to retry delivery.Legitimate mail servers automatically retry after a short delay, and the message is delivered on future attempts. No action is required on the recipient’s side.If a message never arrives after the retry period, the sender should check their mail server logs for delivery errors.
This error means the connecting IP address has been blocked by MailChannels. The sending server cannot deliver mail until the block is resolved.The sender should contact MailChannels support to request a review of the block.
This error means MailChannels was unable to resolve the sender’s domain — the DNS lookup returned an NXDOMAIN (domain not found) response. This can be a transient DNS issue, or in the case of newly created domains, the domain may not have fully propagated yet.The sender should wait and retry. If the problem persists, contact MailChannels support.
This error means the domain is not authorized to use MailChannels Inbound Filtering. MailChannels received a message for a domain that has not been provisioned on the service.If you are a hosting provider, make sure the domain has been added to your MailChannels account. If you are a domain admin, contact your hosting provider to confirm that inbound filtering has been enabled for your domain.
This error means the message was identified as abuse — spam, phishing, malware, or another policy violation — and has been permanently rejected. The message will not be accepted or retried.
This error means the message was rejected because it was identified as spam, phishing, malware, a virus, or another form of abuse. The message will not be accepted or retried.In some cases, this notification includes a link. If present, the sender can use it to release the message from quarantine directly.
This error means the message was identified as spam and has been permanently rejected. The message will not be accepted or retried.If the sender believes their message was incorrectly rejected, they should contact MailChannels support for assistance.
This error means the message contained a file attachment with a blocked extension. MailChannels rejects messages with certain high-risk attachment types to protect recipients from malware and phishing.The sender should remove the attachment or use a safer alternative format. See Attachment restrictions for the list of blocked file types and guidance on alternatives.
This error means the message exceeds the maximum allowed size. See Attachment restrictions for the size limit and guidance on alternatives.
This error means the message was rejected because the sending server is not authorized to send email on behalf of the sender’s domain. SPF (Sender Policy Framework) is an email authentication standard that lets a domain publish a list of IP addresses permitted to send mail in its name. When an incoming message is sent from an IP address not listed in the domain’s SPF record, MailChannels rejects it.This is a permanent rejection — the message will not be retried.If you are the sender, check that your sending IP address is included in your domain’s SPF record. For help diagnosing SPF failures, contact your DNS or email administrator.
This error means the message was rejected because it failed the sender’s domain DMARC policy. DMARC (Domain-based Message Authentication, Reporting & Conformance) allows a domain owner to instruct receiving mail servers to reject or quarantine messages that fail SPF or DKIM alignment checks. When the sending domain’s DMARC policy is set to reject, MailChannels rejects non-compliant messages on the domain owner’s behalf.This is a permanent rejection — the message will not be retried.If you are the sender, check that your messages pass SPF or DKIM alignment for your domain. See DMARC enforcement for more detail on how MailChannels applies DMARC policies.
Each recipient on a domain can have their own spam handling policy — for example, one recipient may quarantine spam while another rejects it outright. When a single message is sent to multiple recipients with different policies, MailChannels cannot apply two different actions to the same message in the same SMTP transaction.In this situation, MailChannels accepts the message for recipients whose policy matches the first recipient it processed, and returns a 452 4.5.3 Recipient policy mismatch response for the remaining recipients. The 452 code is a temporary deferral, not a permanent rejection.Well-behaved mail-sending applications automatically retry deferred recipients in a separate SMTP transaction, so the message is ultimately delivered to all recipients — possibly with a brief delay. This behavior is transparent to most senders.If a message has not been delivered after the retry period, try sending it to recipients one at a time. If the problem persists, contact MailChannels support.