Block Outbound Port 25 SMTP Traffic Cisco Catalyst Switch ACL

Log into to your switch, enter "enable" mode, then "configure terminal" mode:

access-list 199 deny tcp any any eq smtp
access-list 199 permit ip any any

Now, on any interface that you want to deny outbound port 25 SMTP traffic to:

ip access-group 199 in

Write out the changes and you're done