SuSE-FW-DROP-ANTI-SPOOFING

hello, I have problems to contact my firewall from outside. Our uunet router is capable to contact our firewall and some of the services provided internally from outside. but when I plug a local pc with the same IP address to the FW_DEV_EXT network and try to connect to smtp (for example) I get SuSE-FW-DROP-ANTI-SPOOF IN=eth0 OUT= MAC=... SRC=<router> DST=<firewall> LEN=48 TOS=0x0 0 PREC=0x00 TTL=128 ID=546 DF PROTO=TCP SPT=1043 DPT=25 WINDOW=16384 RES=0x00 SY N URGP=0 OPT (020405B401010402) again: the uunet router is capable to send mails via smtp port and I do not get the above error message. Why do I get those messages?