Broadback wrote:

An error occurred while sending mail: The mail server sent an incorrect
greeting: Your IP address is on the XBL blacklist! Sending denied.
please see http://www.spamhaus.org/query/bl?ip=80.229.179.189.

It looks like you're trying to send email direct from your PC, rather
than via Plusnet's email server, not many places will accept email from
IP blocks that are adsl or dialup (as that IP address indicates) and
especially if someone else within the same block has been sending spam
and got the block blacklisted.

If you configure thunderbird to use relay.plus.net you should be ok.