Hi
Looking at this issue it appears this message has been filtered as being possible spam.
In order to have this investigated please follow the processes laid out in the link below:
https://support.symantec.com/en_US/article.TECH82881.html
Alternatively you can contact the recipients via a different medium and request them to open a Support case to have this investigated.
Thank you
Ian Tiller
Tier 2 Senior Technical Support Engineer