I think someone is playing a practical joke on me and is sending emails from a hotmail account to various people I know. I went to Message Options/Internet Header for emails I receive from the person I am suspecting and got the same information from a friend receiving these other emails (also from hotmail but under different email address.) If the x-originating-IP is the same for most of those emails I receive, is the same person sending these emails? Sometimes it is different and I suspect they are sending the message from a phone for those..those x-Originating-Ip are always changing.
If I had 5 hotmail accounts and sent emails to you from each of those accounts and was on the same computer connected to the same network each time I sent the email the X-originating IP would be the same on all emails.
It's very easy to spoof email addresses however, it's not easy to spoof IPs.
So the short answer to your question, is the IP in the X-originating IP that of the person sending the email. Yes....in most cases. However, it's not 100%.
People often wonder how their email addresses get out. You could've sent an email to all of the people they're sending email to. Then someone on that list fowards that email and so on. Somewhere down the line someone is collecting those email addresses. Perhaps this is the case here. They read back through the email, acted as though they were you and sent an email to the same people in the 'To:' section. I always tell people to BCC when sending jokes, or something else that has the chance of getting forwarded to others.