User A shares one of his calendars with user B and gives her/him modify and/or creation and/or deletion privileges on it.
When user B creates/modifies/deletes an event with invitations in A's shares calendar, an email is send to all invitees (= iMIP).
That email informs the invitees about the change made to that event.
For invitees not part of the same SOGo server, those emails are the only way to get notified about such a change.
Therefore those emails are essential.

This email is sent with user B's credentials, as she/he has made the change.
But that email has a FROM containing user A's email address as owner of that calendar and therefore that event.
That is correct, as accepting/denying that event by the invitees must go back to A.

But as this address does not belong to user B, our email system is denying that email.
Currently we have to manually set exception rules for user B, to be allowed to send emails with FROM address of A.
That is a security risk, as user B can send now any email in the name of A.
This is also a hassle and error prone manual workaround.

Therefore it would be better, if such system emails would be send via the same account used for email alarms already.
That account is already able to send as any user and SOGo knows and uses it already.