I use Thunderbird (78.11.0 (64-bit)) as my mail client and I'm posting from my own domain, as I have done for the last 15 years. I've recently started getting this message too. I've had one supplier email me on several occasions but I cannot reply back because I keep getting this message. Today I received a reply to an email I'd sent and when I tried to reply I got the same error message. I can't run a business where I can't communicate with my customers.
I haven't changed any of my settings and some mails are going through so why is this happening? Virgin Media, I need you to sort this out asap please.