No mail, but using port 25

[Mike - EMAIL IGNORED <m_d_berger_1900@xxxxxxxxx>]

On FC4, for my own edification, and hopefully not
doom, I have set iptables to pass only packets
that I understand, and to log rejections.  I find
port 25 in use, despite that I am not using mail.
What is the purpose of this?

Thanks for your help.
Mike.