FIDO prohibits any commercial or illegal traffic. The first is prohibited because the system is all volunteer, and it is not fair to expect sysops all over the world to handle commercial advertising or solicitation for free. Illegal traffic includes information on software piracy, the names and phone numbers of so-called 'pirate' BBS systems, and similar things. Even though sysops almost never read the mail that passes through, they could be held responsible for forwarding illegal traffic, so FIDO prohibits the transmission of encoded or encrypted messages, no matter what. On Internet, there are few official rules that apply across the entire network.
Most connected systems implement policies of their own.
Usually this means wasting the time or money of sysops and callers. It is informative, and often amusing. Go ahead and experiment with this system. Remember that anyone at all can receive Netmail or E-Mail here, whether a subscriber or not.