Server: unauthenticated access

[Valentin-Kaiser]

If the server requires authentication, e-mails can only be sent with the correct credentials.
However, it is also possible to omit authentication. In this case, the server allows the message to be sent even though authentication was actually required. There is a variable in the connection structure that is set after successful authentication, but this variable is only used to prevent double authentication.

type Conn struct {
...
	didAuth bool
}

The commands like 'DATA' should first check in the handle function whether the authentication was successful.

switch cmd {
case "DATA":
	if !c.server.AuthDisabled && !c.didAuth {
		c.writeResponse(530, EnhancedCode{5, 7, 0}, "Authentication required")
		return
	}
	c.handleData(arg)
}

[emersion]

Duplicate of #216