Fixed threading of messages where the Message-Id header does not exist
Fixed threading of messages where the Message-Id header does not exist

jstedfast/MailKit,dsbenghe/MailKit,mganss/MailKit,sstraus/MailKit,matt2003/MailKit,nachocove/MailKit,rajeshwarn/MailKit,eino-makitalo/MailKit,jamie-dainton/MailKit,FubarDevelopment/MailKit