smtplib error: Unknown Protocol for Automated Outlook Email with Attachment

I'm trying to send a basic email with an attached document. The example I have works fine with a google address smtp.google.com, but when I try changing it over to smtp.office365.com, I get this error: [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:852).

This is a domain email that runs through the office365 SMTP server as a host for the email service. I've checked with the IT team, and they have turned on SMTP authentication on the account.

Obviously, this has been done before, so I have checked my code against this example, but don't see any obvious differences that could be causing it. I've also doubled checked the smtplib documentation, and smtp.office365.com is a valid, recognized SMTP address.

I've written this out as follows (note the confidential credentials which prevent a minimal reproducible example). I've noted where the error occurs, it's almost like smtplib is not recognizing the office365 SMTP address.

    def send_email(self, html_message):
            # Create a text/plain message
    
            formatted_date = datetime.datetime.now().strftime('%Y%m%d')
    
            msg_root = MIMEMultipart('related')
            msg_root['Subject'] = self.client.client_name + 'Test Forecast'
            msg_root['From'] = self.config.get_email_from
            recipients = self.client.recipient_email
            recipients = recipients.split(', ')
    
            body = MIMEText(html_message, 'html')
            msg_root.attach(body)
    
            filename = self.client.city + ', ' + self.client.state
    
            # PDF attachment
            if self.client.use_pdf:
                filepath = self.config.get_email_pdf_file_path
                fp = open(filepath, 'rb')
                att = email.mime.application.MIMEApplication(fp.read(), _subtype="pdf")
                fp.close()
                att.add_header('Content-Disposition', 'attachment', filename=filename + '.pdf')
                msg_root.attach(att)
    
            # SSL port config
            port = 587
    
            # Create a secure SSL context
            context = ssl.create_default_context()
    
            # Connect
            with smtplib.SMTP_SSL("smtp.office365.com", port, context=context) as server: # Error on this line
                server.ehlo()
                server.starttls()
                server.login(self.config.get_email_from, self.config.get_email_password)
                print("Sending email report...")
                # Send email
                server.sendmail(self.config.get_email_from, recipients, msg_root.as_string())
            print("Your email was sent!")

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