TCP SYN RES SET what does this mean

I am getting a security alert from my firewall about twice a day that looks like:
2003-05-01 08:35:01 – TCP SYN RES SET – Source:209.47.197.15,40192,WAN –
Destination:<>,25,LAN

The firewall is a Netgear FM114P

I am running ansmtp server at this address and it seems to be working fine.

My question is: What does this message mean?