I use Thunderbird for three email accounts of which two, including the one that will not send an email, use my provider, 1and1.co.uk. In addition, I can send mail on the problem account from my desktop and KIndle with no problem. This laptop is a Lenovo Think Pad Edge now running Windows 10 and updated in 2015 from Win 7 with no issues then for Thunderbird. I run Avast on both laptop and desktop without issue having returned to Avast from AVG relatively recently and having before that, used again Avast. I am unable to associate the failure of this laptop to send emails with anything that has occurred e.g. update of software; this may be the case but I am not able to identify. Window 10 Pro Avast Version 11.2.2262 regards rob