Outlook maintains the AutoComplete list. The list is used by both the automatic name-checking feature and the automatic completion feature. The AutoComplete list, also known as the nickname cache, is generated automatically when you send email messages from Outlook. The list contains SMTP addresses, LegacyExchangeDN entries, and display names for people to whom you have sent mail previously.
Outlook limits the number of entries that you can save in the AutoComplete list. After you reach this limit, Outlook uses an internal algorithm to determine the best names to remove from the list. It does this based on a usage weighting. Therefore, you may find some names unexpectedly removed from your nickname cache. There are two general approaches that you can use to avoid this situation:
The limits are as follows:
- Outlook 2016: 1,000 entries
- Outlook 2013: 1,000 entries
- Outlook 2010: 1,000 entries
- Outlook 2007: 2,000 entries
- Outlook 2003: 1,000 entries
Increase size of AutoComplete list:
1. Open the Registry Editor and navigate to:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoNameCheck
2. Create a new key and value into the Registry key of Outlook: MaxNickNames
3. Enter a decimal value (6000 - 10000)