There’s another post with the same issue:
" Google Calendar Sync Issue – Accepted Invites in eM Client Not Syncing to Google Calendar"
However the solution of changing Google Calendar settings to “From Everyone” shouldn’t be an acceptable solution. It’s a serious security hazard allowing any and all bad agents to add calendar invites for phishing. It also validates emails to the attacker.
I’ll reiterate the issue we’re seeing across our org:
Invites from outside of our domain (We’re using Google Workspace) do not get added to the google calendar if the event invitation setting is set to “When I respond to the invitation in email” nor “Only if the sender is known”
It works as expected if you accept the invite in Gmail, Mac Mail, or in Outlook but not in eM Client.