HELP ME( Router problem)

Hello.Our smtp mail server has some router problem. But im not sure this problem is in network or filtering server. so i'm asking to you.in sort speaking our smtp server is domino and the server is relaying to filtering spam server. Our filtering spam server is working double system. for instance we use L4 switch to take over when one system is shutdown automatlly.The problem is L4 is checking filtering server each 10 second and if some server no responde port 25, L4 try to recheck probe helth check 3 times. if a filtering server still don't response L4 probe dicide filter server's 25 port failed.At that time our domino server can't route to L4 anymore.When i put to domino console , Server give us below infomation=====================================Msgs State Via Destination147 Retry( 2) SMTP 156.xxx.xxx.xxx (Push)Last error: The server is not responding. The server may be down or you may be experiencing network problems. Contact your system administrator if this problem persists.Next retry: 2008-03-26 오전 10:15:04=====================================And below log is L4 log=====================================Mar 26 09:34:45: %CSM_SLB-6-RSERVERSTATE: Module 8 server state changed: SLB-NETMGT: TCP health probe failed for server 156.xxx.xxx.xxx:25 in serverfarm 'NOS-25-NAT'===================================== and our domino smtp server can't route to 156.xxx.xxx.xxx any more and so there are many pending mail in mail.boxBut strangely when i check our filtering server's status at that time, the filter spam server was no problem i mean the server was processing so many mail. most of wondering is there possible if filter server is no problem , why L4 probe decision the filter server's 25 port failed. so i guess, probably L4 probe have wrong dicision.so domino smtp server also continusly never route to L4so i've made some program document in the server, every 5 min automately "route 156,xxx,xxx,xxx". we knew if we type "route 156.xxx.xxx.xxx" the router try to send again and routed well.Please help me. do you have any exprience like that?if you have any question about my topic, please let me know.Thank for reading.