I used to get randomness like this back in the day I had a crash or forced quit of the application, otherwise it kinda seemed liked it just worked.
If you have time machine (or some other backup) you can try restoring that (after taking a copy of your mail as at today for good measure).
Long ago I bought myself my own domain and hosting so I didn’t have to deal with the ISP emails or the pain that is caused when you want to move ISP’s.
In the olden days that was fine, Outlook just POPed the mail down onto my computer and problem solved, no worry about limits. As time moved on and smart phones became a thing I realised I wanted my email with me so had to look for a provider that could keep it all in online accessible box of some sort.
I used Hotmail/Outlook for a while then switched to Google. Both support picking up mail from multiple different boxes (or you just set a forward on the server side for domains you control), you can also send mail through your custom domains from within Google as if they are coming from [email protected] rather than [email protected]… of course this means selling your soul to Google (which you can do even if you keep iiNet) plus the cost of your own domain if you want that freedom. Being able to have my mail on my phone or work computer in all it’s three decades of hoarding glory if just nice 
There are other options like Fastmail which is supposed to be very good and protect privacy, but that one comes at a cost.
