Hello, we have to have a closer look into the actual MailMessage so that we could help you with this issue. Please use the MimeOption.DoNotParseMimeTree when loading the MailMessage from the database and then save the loaded mail into a file like that:
MailMessage message = new MailMessage();
message.Options = MimeOptions.DoNotParseMimeTree;
Then please zip the file and mail it to email@example.com for analysis. We should be able to help you.