I wonder why sender and recipient addresses are sent twice. Does the second time just show up as email content?
The first time it is sent, it is for the benefit of the email server. The second time is for the email "From" in your email message. Note it contains your name, so it shows from "Me" in the From field.
So if I replace the sender email with say another person's email address, say Mr. X, will the recipient think that the email came from Mr. X? Kind of dangerous?!
Not dangerous unless you are easily fooled. Spammers use this all the time. I get spam mail from me.
helo is email server for Hello.
There is no login. No user. No password. This does not send email like an email client. This sends email like an email server (or a spammer!).