Message " Socket Error 6001 Port 587 Specified File Could Not Be Decrypted"?

I recently changed Internet providers, from Primus to Bigpond, and suddenly my Incredimail programme ( which I have used for 15 years or so) has started giving me the message " Socket error 6001 port 587 specified file could not be decrypted"After much time spent trying to get it to work I downloaded Thunderbird, put all the necessary specs in and it works fine. With exactly the same specs, pop smtp, ports etc in Incredimail, it still gives me the same message.It's very frustrating, and yes I have gone right through Incredimails Help pages, but in the end it just tells me to contact the Internet provider, who tells me that they because Incredimail is not one of their supported programmes.