Well, with a PIX 515E firewall, everything is blocked unless you tell it to allow it. If stuff is coming in and out through port 25 which is email (SMTP), then there is an access-list or something allowing SMTP or port 25. Look for that in the PDM or command line and see.
The PIX by default blocks all incoming traffic but allows all outgoing traffic. If you need to block incoming and outgoing SMTP traffic, then you need to configure an ACL. If you have an access list already configured and applied to the outside interface, make sure it is not permitting SMTP traffic, this ACL controls incoming traffic. For outgoing traffic you need to configure an ACL and apply it to the inside interface, that is is one is already configured. The link below is an excellent guideline for your task:
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