pdf attachments removed from Outlook outgoing emails

Folks, I have been through as many forums as I can and can't seem to find an exact match to my problem.  I am emailing invoices as pdf's out of Reckon Accounts 2016 through Outlook 2016 (downloaded to PC from Microsoft 365 on Win10).  I can see the emails in my sent items with the attachments but all of my customers are saying they get the email but not the attachment.

I know there must be a setting somewhere in my Norton Internet Security to allow for the Reckon/Outlook emails to go out with the pdf's but I can't find it.  Can anyone help or point me to the correct forum?

 I can provide more information if needed.

Adam