Setting Message Importance using SMTP Mail

[Whyatt]

Hi all,

I'm trying to create a message using SMTP Mail through Python with a
message Importance of either 0 (Low) or 2 (High).

If I do outer.Add_header('Importance', '0') it is ignored.

If I do uter.Replace_header('Importance', '0') I get the error below.
Traceback (most recent call last):
  File "#####", line 840, in <module>
    outer.Replace_header('Importance', '0')
AttributeError: MIMEMultipart instance has no attribute
'Replace_header'

Can anyone provide any clues on the correct way of updating the Msg
Importance header?

Thanks in advance
Ed