eMail error when deploying

This is probably minor, but one of my servers seems to have an issue sending mail when a deploy starts. The error pops up after asking for the vault password on a deploy as well as at the end of the deploy:

Ubuntu 22.04.4 LTS
Traceback (most recent call last):
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/manage.py", line 190, in <module>
    main()
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/manage.py", line 48, in main
    execute_from_command_line(sys.argv)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/corehq/apps/hqadmin/management/commands/send_email.py", line 45, in handle
    mail_admins(subject, message, html_message=message if is_html else None)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/mail/__init__.py", line 104, in mail_admins
    mail.send(fail_silently=fail_silently)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/mail/message.py", line 284, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/mail/backends/smtp.py", line 109, in send_messages
    sent = self._send(message)
  File "/home/cchq/www/monolith/releases/2024-05-11_18.43/python_env/lib/python3.9/site-packages/django/core/mail/backends/smtp.py", line 125, in _send
    self.connection.sendmail(from_email, recipients, message.as_bytes(linesep='\r\n'))
  File "/usr/lib/python3.9/smtplib.py", line 908, in sendmail
    raise SMTPDataError(code, resp)
smtplib.SMTPDataError: (501, b'5.6.0 Request body validation error. Need either non-empty Html or PlainText body to be present.')
Connection to 10.3.0.4 closed.

It's worth noting that we've recently switched to Azure Communication Services to send mail (SMTP).

Also to mention, the "deploy successful" email goes off without error and Commcare is able to send general user mails without issues (user creation, password reset, etc.) so it seems it's only at the start of the deploy that it's failing to send mail (doesn't affect the deploy).