The Message-ID header is somewhat special because it should be unique for each email sent. It is also one of the default header fields that is added whenever a new Chilkat email object is instantiated.
The answer to this question involves the mailman.AutoGenMessageId property, which has a default value of True. The online reference documentation provides more information, but I'll reproduce it here:
Once AutoGenMessageId is turned off, the Message-ID header may be added/changed just like any other header field (via the email.AddHeaderField method, which adds or replaces a header field).
answered Jun 14 '12 at 08:52
You're very welcome. I think your custom Message-ID might be invalid because it's missing the "<" and ">" delimiters at each end. For example, try changing your code to this:
Best Regards, Matt
answered Jun 18 '12 at 17:28