Assume that User A sends a meeting invitation to User B, and User B then forwards the invitation to User C. When User C receives the invitation, the mail shows that it was sent by the User B server, but the From field identifies the sender as User A.
This issue doesn’t affect regular email messages. Only calendar items are affected. The sender reputation issues cause the forwarded appointment to be blocklisted. Therefore, the email client sends the invitations to the spam folder.
To fix this issue, install the following updates, as appropriate: