Hi @Marianne ,
Welcome to our forum!
Does this issue occur with other users around you?
And what's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook 2013 to the latest version.
In addition, what's the type of your account(POP/IMAP/Exchange)? And have you turned on cache mode?
As I known, when you search in the Outlook, you can place your mouse over the search results to show where the email is located (in which folder). It's suggested that you could try to check the location of the emails.
On the other hand, to further confirm whether the issue is related to the Outlook client, please try to log in to your web mail and checking if the emails exist. If emails are displayed in web mail, please provide more information about the questions that i mentioned at the beginning.
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