feat(protocols): trusted senders are not filed as spam, when the message proves its sender

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/protocols/internal/application/port/trusted_senders.go

@@ -0,0 +1,18 @@
+package port
+
+import (
+	"context"
+
+	"github.com/google/uuid"
+)
+
+// TrustedSenders is the list of senders a mailbox has said it trusts.
+//
+// Mail from one of them is not filed as spam, which is what makes "this is not
+// spam" mean something: before, moving a message out of Junk taught nothing and
+// the next reply from the same person went straight back.
+type TrustedSenders interface {
+	// IsTrusted reports whether the mailbox trusts this address, which is
+	// already lowercased.
+	IsTrusted(ctx context.Context, mailboxID uuid.UUID, address string) (bool, error)
+}