Suddenly, possibly due to an update but unsure (started on 6th November), my emails no longer sent in Thunderbird due to the above message. I am still receiving emails in Thunderbird. I have checked all of this with plusnet and as I can send and receive without problems in webmail, this is being seen as not a plusnet issue. In terms of trying to resolve, I have tried adding my password in to the box, have tried checking all of my smtp settings against those advised to no avail. Has something changed... There seems to be many similar threads but not sure if mine is the same so creating a new thread.