This bug is similar to bug 6302 or perhaps has reproduce again. If there is not a to_address at messages_queue avoids to send any mail, to any other user at message_queue I think desirable behaviour should move to 'failed' thos without to_address and continue with the next one of the list tested in koha 18.11.03 debian 8
*** This bug has been marked as a duplicate of bug 28803 ***