By default Microsoft Outlook supports up to 1000 autofill entries. Beyond 1000 it will do its best to delete older and unused addresses, however we found this process to be arbitrary. Use the following registry key below or the attachment and value to set it to 1500. This is an unsupported configuration from Microsoft, so it might be best to backup your autofill using NK2edit by Nirsoft.
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoNameCheck]
"MaxNickNames"=dword:000005dc
Attached files: