eM Client error box when trying to send or receive emails

Errror Message box from eM Client : Password required for IMAP  
Authorization Failed.  Authenticate Invalid
But when I try to enter the only password I know, the same error box comes back.  i am not at all technical, so if something is required via Menu  Tools  Accounts  IMAP, i really need simple step by step instructions with details of what exactly to put in - previous FAQ in this website say ‘check your settings are correct’  - i wouldn’t know what is correct and what isn’t!  
My server provider is BT and they say as i am using an external client, and as i can access emails via the (horrible and flashy) bt website, this proves the problem is with eM Client.
Hope someone can help.

The correct settings for BT are:

port = 993
security policy = Use SSL/TLS on special port (legacy)

You will find these in Menu > Tools > Accounts, then click on the IMAP tab for your account.