User Configuration > Administrative Templates > Microsoft Outlook 2016 > MIME to MAPI Conversion
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
By default, the limit is 12288 recipients included for an e-mail message when the message is converted from MIME to MAPI. The number can be set to any positive integer. This helps prevent scenarios in which Outlook hangs while attempting conversion.