By 2003 less than 1% of corporate mail servers were available as open relays, down from 91% in 1997.
You could probably turn it into a nice little file/print/web/mail server.
Are they trying to put mail servers on my system?
It can be done using a mail server with telnet.
It is entirely up to the configuration of the mail server.
In the simplest case, a domain may have just one mail server.
It may in fact be an array of mail servers that share the same host name.
The mail servers around the world still have to deal with all the resulting spam, for one thing.
You could *gasp* do this with most *nix mail servers too!
Control of mail servers remains as it is now.