Emails simply disappear even though they are shown as incoming on the screen.

Hello everyone I work with version 8.2.1721 (3b3cd99) and Firefox under Windows 7. Sometimes I see the message New mail on the screen. so also today. I saw the announcement that a new mail had arrived for about 5 seconds. after that it disappeared from the screen. Despite looking everywhere, I couldn’t find it. It seems that this email doesn’t even exist. I have to assume that this has happened several times, because I’ve been asked (by my customers) again and again if I haven’t received an email from him. I couldn’t believe it until I saw it for myself today.
Is this phenomenon known, or do other people also have this problem?
Thank you for your help

Go to “Menu / Rules” and see if there is any rules setup to automatically move incoming email.

Thank you for your help. I have one more question: when I write an email, I see it under sent items. But I also see them in the inbox. Can I turn this off so that I don’t have a duplicate of the mai I sent in my inbox? If I see that in the sent folder, that’s enough.

when I write an email, I see it under sent items. But I also see them in the inbox

1). You may have the default “Conversation view” enabled which will then show your sent email under your inbox messages.

To disable conversations go to “Menu / View / Conversations / Disable Conversations”

You can also disable conversation view in Settings via “Menu / Settings / Mail / Read / Conversations” and select “Disable Conversations”

2). Another reason sent email can appear in your inbox is that you might have a rule setup which copies the sent email to your inbox. So go to “Menu / Rules” and open / edit any local & cloud rules to check.