email account SMTP settings in thunderbird probably… this ip doesn’t seem to host any services.
looks like a typical ISP ipv6, so you’re probably using your own pc/mobile as SMTP, not the correct MX host.
so check that first…
Thunderbird account settings → Outgoing host (SMTP) and choose/add your proper SMTP host details.
could also be issues with firewall, hosts file, et others…