Seems like a number of folks are having issues with Mojave and Apple's Mail program.
Up until about 2 months ago, I was using Outlook 2016 as my EMail program. But then it gave me problems, as I could not "get to" any of my EMails (even ones stored on the database Outlook 2016 maintained on my machines (I was able to see my new EMails on my ISP's (Comcast) site))). So, I decided to switch to the excellent free EMail program Thunderbird. Here is the link for it:
The only issue I initially had was that with the way Outlook 2016 "stores" EMails, I could not directly "import" all of them into Thunderbird, and I needed to purchase the excellent program OLM Extractor Pro for $49 to assist with that transfer (I believe with Apple'a Mail program, you should not need that program, but you should research that first).
In any event, Thunderbird works fine for me (I've also been testing it with Mojave, and as expected, it works well).