I can see that localhost:25 is by default used after an install on many Linux distributions.
Why is this for?
My hint is for to deliver local system-generated mails?
The reason that by default a SMTP server is up and running in a new Linux install is for the system to deliver important notifications and other information (status of cron jobs, etc.) to the root user.
Don't worry however -- for security reasons, the SMTP server is configured to accept mail from localhost only, so not to be an open mail relay.