Most email servers that are on an ISP only allow use when you are on their network. So if you change providers you can lose access to that old email address.
Yep, people really get upset about that.
OK, P, I have been unable to respond to the previous thread. My current e-mail address ends in "dot com" and was set up when we changed our internet provider. I think mail was set up on my old address and old internet provider. (I am now thoroughly confused and really don't remember just what I did to cause this delimma.) When I did as you suggested, I got iCloud and Yahoo mail and my Apple Mail just disappeared. When I do a search on my Mac, it shows a list of some things that were there. It was my main mail, and I don't want to have to notify everyone that I have a new mail box. I will monitor the forum for answers should there be any. Thanks, Old Granny Lola