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 50 embedded messages when an e-mail 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.