I use Yahoo mail. Yes, I know, but I have used it for a very long time, and it works.
But over the last month or so, my messages at the bottom of my inbox have been disappearing. My out box still is there, with all the messages, going back years.
Now, I do cull my messages quite often, and the mailbox isn’t close to full- unless Yahoo has changed the size drasitcally recently.
In my case, it may be because I’m not cleaning the folder enough. I belong to a mailing list and get several emails a day from it so I have 1200 that I haven’t read yet. The new Yahoo interface is a royal pain in the ass because it starts having problems if you do something so bold as flip from one message to another quickly. Add in images and it’s literally impossible to clear the backlog. So Yahoo has started silently doing it for me. It’s annoying that they offer “unlimited” storage until you become a burden to their mail server at which time your messages can and will be deleted without notice.