Dear Dennis! Fine to see your note, I have exactly the same issue with certain (non-TEK/iogroup) contacts.
A few notes of my experience:
Worst email domains are Yahoo and Free.fr,: The correspondent can send to me but ever reply is rejected with multiple notices sent periodically as you mention.
The correspondent never has any clew what to do and even if you try to whitelist its never effective.
I believe that the offending email servers have a hardware antispam filter with blacklists updated dynamically.
The issue can be either your IP address or that of your email server is on the BL, OR that your email server domain is on the BL.
For years I tried to fix this both in France and USA, to no avail!