My correspondence keeps bouncing, blocked by their 'Barracuda' software
Your only solution is to use the telephone.
You are going to have to deal with the Barracuda problem as another issue. Your ISP decided to give you a tainted IP address. Your IP address is black listed on the web because it was used by a Spam generator. You are going to have to convince your ISP give you a different IP address - which would be my choice. Or contact companies like Barracude: https://www.barracudanetworks.com/
go thru their appeal process to get your IP address removed from the black lists. I have no idea how many companies and black lists there are.
This situation is kind like getting the old telephone number for someone like Justin Beiber after it gets spread across the internet. It takes a long time for the web to realize Justin changed his number.
Unless stated otherwise, all content in this post is My Personal Opinion.