Remote SMTP mail Auth failing

Here's an odd one, some smtp mail which should be accepted by the server as outgoing mail for a users account will not make it from specific locations. Some locations just work, others just do not, I noticed one Spamhaus rejection on one location trying to send mail but no others even though the server just drops the connection after a while. I am wondering is this a rogue blacklist contained in the server ??? This is a new problem never had it before, server has been running for over a year no problem... OS X Server 10.4.11 X Serve 4 GB Ram