Appontment created in D365 not synchronized in the Outllok calendar of the D365 user appearing in the Appointment field "Required"

Hello, 

the user created and sent an APPOINTMENT from D365 to a colleague (also D365 user). The colleague, who is apparing in field "Required" cannot see the APPOINTMENT in his Outlook calendar.

Both users has App for Outlook and security role Dynamics365 App for Outlook.

thanks in advance

Sil