My email was blocked by BT because it had been hacked. They have reset it so that it now works on the Yahoo webpage, but I cannot get it to work on eM Client
First make a backup using Menu > File > Backup, then go to Menu > Tools > Accounts and remove the email account from eM Client then add it again.
Thanks Gary. Before I saw your advice I had tried doing something very similar to what you suggested and now it is working OK again. Thanks very much for your help. Much appreciated. Adrian