I use Mac OS calendar on daily basis. This is the most popular solutions. Native mac OS calendar app or Fantastical (https://flexibits.com/fantastical). This gives a lot of benefits + cloud sync between differen macOS devices.

I am a Postbox user which wanted to migrate to the em Client, but... because of lack of this feature - i cannot use em client. I have a lot of meetings invitations which i need to save into macOS calendar.

Saving the file locally on a hard drive and then importing them to macOS calendar - it is not comfortable.

The whole issue came up here:
https://forum.emclient.com/t/support-for-ics-files-into-calendar/35868/28

And this is the post where i described how this usecase looks like on EM Client vs Postbox
https://forum.emclient.com/t/support-for-ics-files-into-calendar/35868/19?u=chandler