In Ubuntu the server settings aren't under tools - on the top toolbar click 'Edit' then 'Account Settings'.
That's where I have been going.
Default setting for port without SSL/TLS is 25 - Don't work
With SSL/TLS is 465 - don't work
server - smtp.mail.yahoo.com - don't work
I've removed the account and put it back several times.
When setting up account
IMAP becomes - Pop3.btinternet.com Pop 110 None and is ORANGE
SMTP becomes SMTP.mail.yahoo.com SMTP 465 SSL/TLS and is GREEN
Server settings - POP 3 btinternet.com security - None
Outgoing server - smtp.mail.yahoo.com - Port 465
Receives mail but won't send