Why doesn't my Lumia 640 allow me to access my emails after I've read them?

[Windows Central Question]

As new emails come in, I get the notification and can read them, but after that, they disappear. Inbox just says "no messages", as do all the other folders. "All", "unread", "urgent", it makes no difference.

There is nothing obvious in "settings" that seems to be responsible, and my email account is working as normal on my laptop.

Any suggestions?