There is some that don't know their email systems so I'm going to write that if you use IMAP then it's true the content does go away if you don't transfer content from one IMAP server to the next. This is well discussed so I'll stop here with a link.
After switching to a new hosting co., the Mac Mail on my iMac works fine, but everything seems to be lost on my wife's MacBook. I entered all the supposed correct new server info several times and still nothing. Both my hosting support and Apple Community Forums are closed down, and I no longer have AppleCare. What can I do?