Recently some emails are getting stuck on queue to be sent out. But it happens with a few receiving domains outside only. When we send emails out, they are stuck in queue, then stay for about certain time, 30-60 minutes, then evetually go out. When I freeze the emails and force to delete message with NDR, it comes out as following. I thought it's receiving email server's spam filter, but since it happens to other a few domains, I think there might be some settings are not right at my side, too. Or maybe I just discovered other problems we had before because of the first problem I found;
"This message was rejected due to the current administrative policy by the destination server. Please retry at a later time. If that fails, contact your system administrator."