Email problems with Live Mail and Thunderbird

WE are having problems with our farm emails - we receive them OK, but we can't reply or send new nessages. We were using Windows Live Mail but when that started playing up we were advised that as microsoft no longer supported live mail, we needed a new email programme. So I downloaded Mozilla Thunderbird. But that hasn't helped - we still can't send emails on our farm account.It doesn't recognise the password with our ISP's outgoing server so the connection fails. Thank goodness we also have gmail and Facebook which allow us to keep in touch.