My replies to a support ticket email are bouncing - " - address not found"

I replied to a support ticket email that I’ve replied to before successfully.
Today, my reply is rejected. I get a reply from that says:

Address not found.
Your message wasn’t delivered to because the address couldn’t be found, or is unable to receive mail.

The response from the remote server was:

550 5.1.1 <[]: Recipient address rejected: User unknown