How do I stop Outlook/Office365 from changing a header?

Question

I am using the Python library smtplib to send Email messages with Office365 as the SMTP server. Everything was fine until a few days ago when my From: header seemed to not be processed. The Python code I am using:

import smtplib
from email.mime.text import MIMEText

def login():

    server = smtplib.SMTP(mail.mail_server, mail.mail_port)
    server.starttls()
    server.login(mail.mail_username, mail.mail_password)
    return server


def send(subject, body):

    msg = MIMEText(body)

    msg["From"] = mail.mail_from
    msg["To"] = ", ".join(mail.mail_to)
    msg["Subject"] = subject

    server = login()
    server.sendmail(mail.mail_username, mail.mail_to, msg.as_string())
    server.quit()


send("test", "test")

What is strange is that if I log onto Outlook or OWA, I can see my header if I view the sent message details. I sent a test header of From: xxxx <info@...>

However on the recipients inbox message, the header is simply the Office365 User's name and the info address as shown above (From: Name <info@...>). The xxxx custom header is gone.

What can be causing my header to be dropped?

Answer 1

While it may not be an ideal solution, I ended up setting up a new email account with the name I wanted and used that, instead of trying to force the From: header.