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Email quarantine and rejection

Email quarantine and rejection are two ways a receiving mail system can handle a message it considers suspicious or unacceptable. Quarantine usually keeps a message out of the normal inbox for review, while rejection refuses the message so it is not accepted for delivery. The exact experience depends on the receiving provider.

What happens when a message is quarantined?

A quarantined message may be placed in a spam folder or a separate review area. An administrator or recipient may be able to release it, depending on the service. Quarantine is not proof that the message is malicious: legitimate mail can be caught by mistake, and some systems quarantine mail because of local rules unrelated to the sender's domain policy.

For example, if a sender's message fails a check of its sender or signature, a receiver might route it to spam rather than the inbox. The user may still find and review it. A different receiver might accept the message or reject it, because filtering decisions depend on the provider's policies and other signals.

What does rejection mean?

When a mail-transfer server (an SMTP server) rejects a message during delivery, it returns a failure response to the sending system. A permanent 5xx response normally tells the sender not to try sending that message again without a change. A temporary 4xx response asks the sender to try again later. A rejection after the sender has handed the message to another server can result in a failure notice returning to the sender, rather than a visible error on the recipient's screen.

DMARC policies can request quarantine or rejection for messages that fail domain authentication, but they are not the only reason a provider might filter or refuse mail. A message that passes DMARC can still be quarantined, and a receiver's final handling remains subject to its local policy.

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