add reject from origin domain feature (#375)

Add a new feature to be able to reject email *from* specific domains.

Co-authored-by: Cyril DUPONT <cyd@9bis.com>

pkg/policy/origin.go

@@ -0,0 +1,20 @@
+package policy
+
+import (
+	"net/mail"
+)
+
+// Origin represents a potential email origin, allows policies for it to be queried.
+type Origin struct {
+	mail.Address
+	addrPolicy *Addressing
+	// LocalPart is the part of the address before @, including +extension.
+	LocalPart string
+	// Domain is the part of the address after @.
+	Domain string
+}
+
+// ShouldAccept returns true if Inbucket should accept mail from this origin.
+func (o *Origin) ShouldAccept() bool {
+	return o.addrPolicy.ShouldAcceptOriginDomain(o.Domain)
+}