When we type an email address in Outlook to send an email, Autocomplete feature provides a list of Recent People and Suggestions in the drop down list.The latest update provides up to 5 suggestions. If you want to delete an email ID from other suggestions list then there is no X sign next to it and you will not be able to remove the entry,
If you like the Old Autocomplete format which display’s full list then you can use below registry entry on client PC.
Open the Registry Editor on your PC by clicking on Start -> Search for Registry Editor or by Click on Start -> Run and type regedit.
Find the registry key at location: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options
Create a REG_DWORD Entry DisableAutoCompleteUpdate with Value of 1 as shown below.
Once you set the above registry key it will show the autocomplete entries as shown in below screenshot.