windows email won’t send

My windows email suddenly will not send. I called the ISP and they walked me through the programming, and it was already set the way it was supposed to be, and still it will not send. I called them again and they said the only thing it could be is the antivirus firewall interfering and that I should disable the firewall. That does not make sense, the antivirus firewall has been there and it sent, nothing has changed on the computer except for the fact that now my windows email will not send. I have to go to the ISP website to do my email, which is ok but I do not like having things not work, and I want my windows email to work, I prefer using it than the ISP website email. Help please. Thank you.