There is no "right sender" when sending email like an email server sends to another email server. No username or password required. That is how my sketch code works. However, if your IP is not a commercial static IP, the receiving email server may not accept your email. My code will also tell you if that happens.
Email servers using a port other than port 25 use usernames and passwords to insure you are a customer of the ISP that hosts the server and not a spammer.