nmap (Windows version) erroneously detects smtp and pop3 port open (Bug?)

hello all,

Can someone explain me this strange behaviour:
From the windows XP command line, i run nmap (the windows version 3-7)
to check opening ports on my linux server:
C:\Program Files\nmap-3.70>nmap -sT -P0 -p 25 192.168.1.5
Starting nmap 3.70 ( http://www.insecure.org/nmap )....
Interesting ports on spw2 (192.168.1.5):
PORT STATE SERVICE
25/tcp open smtp

But:
1) on an Ethernet sniffer, i only see the tcp SYN pkt FROM Windows TO
Linux, but the Linux server NEVER replies
2) running netstat on the linux server doesnt show any Smptp port open
3) the same result occurs wih the pop3 port

Because the linux server doesn't reply to XP, i would like to know why
nmap returns that my port is opened? Is this a bug in the windows nmap
version?

Thank for Your help.
Regards.

(tosh_e800@yahoo.fr)