To specifically allow only TCP 25 traffic to go in/out of your internal network (fr email server), make sure your access-list (ACL) specify this correctly.
From inside to outside, you can have at least the following ACL entry (example):
access-list inside permit tcp host 10.10.10.10 any eq 25
access-group inside in interface inside
Optionally, if you need to allow external email server to send email traffic into your internal email svr, you need to map your internal server with a public IP, and locked incoming access via acl allowing smtp 25(tcp).
access-list outside permit tcp any host 192.168.1.10 eq 25
To log all internal stations session, this is possible if they used Inside interface as gateway, and you have acl logging all permitted or a combination of permit and deny entries. Check the log entries, as well as connection status.
To check connection status, use ' sh conn' command. You may see some internal IP has multiple connections/sessions (via single of a series of ports) to multiple IPs, while some might be multiple session to single IP.
Check the port number and patterns. Some viruses/worms tend to have multiple ports in sequence and can go easily more than 20 sessions/connections attempt (we used the same approached to nail internal PCs spreading sasser virus around).
FYI, PIX has spam/mail relay protection which is enabled by default. Check the 'fixup protocol smtp 25'.
But if the spam/relay is originating from internal PC, you might need to do more. Check your firewall log (destination port number) & connections/sessions for any multiple email sessions from internal host(s) to the exchange server. You might be able to find it.
If you have good antivirus, you could also find the trojan/worms (and pc) responsible for the mess.
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