Calendar doesn't send invitations

Summary

When a user creates an event and invite people, invitations are not sent.

Steps to reproduce

Create an event and invite people to it.

What is the current behavior?

Event is created, but people doesn't receive any invitation.

What is the expected correct behavior?

Each people invite receive an invitation

Relevant logs and/or screenshots

Need to be reproduce on our side to be completed.

Possible fixes

Maybe need to have a look at upstream sources. A similar issue was created a long time ago (here), but it is still opened.