Knowledge base
The receiver refused the connectionError codeThe connection was not established: the remote host rejected it — either the mail server is down, or a filter is rejecting this particular sender.Refusal at the greetingError codeThe receiver returned an error at the very first step of the conversation — usually 4xx: busy, temporarily not accepting mail or not allowing this connection.The receiver went silent mid-conversationError codeThe connection is established, but the receiver stopped responding; the engine waits ten minutes, and this time is not configurable.The name lookup did not complete — and no conclusion is drawnError codeThe name service query failed, usually on timeout; the engine deliberately does not bounce the message on such a failure.The receiver is misconfiguredError codeThe bounce is caused by the remote side’s configuration, not ours.Connection problemError codeThe connection to the receiver failed or was broken.The domain does not existError codeThe recipient’s domain was not found or does not exist. The bounce is about the right-hand part of the address, not the left.The address does not existError codeThe receiver said it has no such recipient. The most unambiguous of bounces: it is about the address, not about the message, not about us and not about the network.Held back over contentError codeRejected because of what is inside the message: a link, an attachment, markup.The receiver dropped the connectionError codeThe remote host unexpectedly closed the connection — the server was shut down or crashed, or this is how it responded to the message content.The mailbox is disabled or expiredError codeThe mailbox once existed, but it was closed, frozen or expired. Formally the address exists, practically it is dead.A problem with the sender domainError codeThe bounce is caused by our domain’s records: they are missing or wrong.The bounce deadline has expiredError codeThe message was not delivered within the allotted time and was bounced on deadline.The host connected and stays silentError codeThe connection was established, no reply followed.OtherError codeA bounce that did not fit any category.Rejected by the receiver's rulesError codeThe receiver refused by its own rules — not over content and not over the address.A breach of the conversation orderError codeA syntax error or a wrong command sequence.The mailbox is fullError codeThe recipient has run out of space. The mailbox is alive, there is just nowhere to put the message.The receiver refused to relayError codeThe remote server refused to accept the message for further relaying.A route could not be builtError codeMail to the recipient’s domain cannot be routed.Held back as spamError codeThe message was rejected on spam signals. The bounce is not about the address — it is about us and about the content.A virus was foundError codeThe receiver considered an attachment or a link malicious.