I reported this issue awhile back and I was advised to ensure synchronisation is on. Valid source of email is getting partially through but a lot of emails are now ending up in SPAM which is only discovered when running GNU/Linux mail client, Evolution.
In Mail app, go to “hamburger” menu, then Manage Folders.
Click on Spam folder, then change Folder poll class to 1st Class.
Also, any folder can be manually refreshed by swiping down from the message list.