[lug] what is port 3 good for?

[Deva Samartha]

something is sending packets out with port 3 and they
get denied by the firewall outgoing to the internet.

the /etc/services says:

compressnet       3/tcp                 # Compression Process
compressnet       3/udp                 # Compression Process

it looks it is the postfix mail demon sending out to
mail servers it is sending mail to.

At one point, I had three packets per second going out and thought
there was a compromise going on and shut the internet down.

Somewhere I read about this being used for a commercial product
checking if email can be transferred compressed.

Is it safe to open this port up?

thanks,

Samartha