bugfix: when dkim-signing submitted messages, use the domain from the "message from header" instead of "smtp mail from"

dmarc verifiers will only accept a dkim signature if the domain the message From
header matches the domain of the signature (i.e. it is "aligned").

i hadn't run into this before and when testing because thunderbird sets the
"smtp mail from" to the same address as a custom "message from" header. but
other mail clients don't have to do that.

should fix issue #22

queue/queue.go

@@ -331,8 +331,13 @@ func Drop(ID int64, toDomain string, recipient string) (int, error) {
 	return n, nil
 }
 
+type ReadReaderAtCloser interface {
+	io.ReadCloser
+	io.ReaderAt
+}
+
 // OpenMessage opens a message present in the queue.
-func OpenMessage(id int64) (io.ReadCloser, error) {
+func OpenMessage(id int64) (ReadReaderAtCloser, error) {
 	qm := Msg{ID: id}
 	err := queueDB.Get(&qm)
 	if err != nil {