I have a problem when sending multipart (HTML & Plain Text) messages. Sometimes the message arrives as text-only. Then I resend and it is HTML. Same code. Sometimes it works.
The MailMan object has a property named LogMailSentFilename. It is a string property that defaults to the empty string.
If you set it to the name of a log file you want it to create, the MailMan will write the exact MIME sent to the SMTP server during the send. You may then check it to see exactly what was sent. If both HTML and text are present, but the email arrives with something removed, then you know that the removal happened after it was passed to the SMTP server. It could have been modified at any point along the delivery route, or by the receiving email client.