Which spam filter setting matters for forwarding addresses?

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If you use a forwarding address to direct mail to a mailbox, the spam filter setting of the the forwarding address is used when mail arrives for that address, and the mailbox spam filter setting is ignored.

For example, if you create sales@example.com as a forwarding address that sends mail to a robert@example.com mailbox, and someone sends mail to sales@example.com, the setting that matters is the spam filter setting for sales@example.com.

The spam filter setting for robert@example.com would not make any difference for that particular message (but would be used if the message were sent directly to robert@example.com, of course).