Disable send when web extensions can't load.

If you enable this policy setting, Outlook won't allow email and meeting requests to be sent until web add-ins are loaded from Exchange.

Supported on: At least Windows 7
Registry HiveHKEY_CURRENT_USER
Registry Pathsoftware\policies\microsoft\office\16.0\outlook\security
Value Nameonsendaddinswaitforload
Value TypeREG_DWORD
Enabled Value1
Disabled Value0

outlk16.admx

Administrative Templates (Computers)

Administrative Templates (Users)