Symantec Email Proxy Deleted Message - why?

Received this message "Symantec Email Proxy Deleted Message" for an email delivering my copy of a UK national newspaper. I've been receiving this paper by email for years.

a) why has it been deleted?

b) why is there no option for me to retrieve the email based on my view that it is perfectly 'safe'?

Further, googling the subject  yields the view that Norton has been doing this arbitrarily for some time without a solution.

This is using all current software: Windows 10 and Thunderbird. Norton 360