E-mail spoofing is the act of making an E-mail appear as if it were sent from a specific person (or address). SPAM and phishing E-mails often use such spoofing to mislead the recipient about the origin of the message.
Most E-mail providers will require authentication (a username and a password) in order to connect to the server and send a communication. Mail servers that do not require username or password can be used to spoof E-mails.
There are not many open mail servers that do not require a username or password. Open mail servers do not usually stay open for long. Offending mail server should be reported to blacklists until the owner of the mail server shuts down the server or requires authentication and restricts persons abusing the mail service from sending mail.