Change the limit for the number of recipients
Supported on: At least Windows Server 2008 R2 or Windows 7
Registry
HKCU
software\policies\microsoft\office\16.0\outlook\options\mail Options
maxnumrecipients decimal
- range ?-99999 Description
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.