Thanks. It's all personal eMail, and post-retirement. This is a separate, personal account that I had. All my work-related eMail was through another account which was passed on to my replacement when I retired.
My former eMail app, Eudora, used to download the mail and hold it on my hard drive, on which I have tons of space. Along the way my former employer, a major Canadian University, switched to Microsoft Enterprise and Outlook. macOS Mail is able to download the mail alright, but it's not a separate copy as Eudora was, which allowed me to delete from the server without losing everything. I guess this just means some hard deleting. I don't know if any other mail app would treat things differently, or if changing to something like GMail would, or is that just another Enterprise equivalent? As you can tell, I'm unsophisticated when it comes to this.