In the `Gitlab::Email::Receiver` we try to find key from [multiple places in the email header](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/email/receiver.rb#L74). That does not happen when a custom address is used for service desk. In that case, `ServiceDeskReceiver` is used and only `to` [is being checked.](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/email/service_desk_receiver.rb#L20). We should check also other keys. That is problematic, when a cc-ed user responds and tries to comment an issue as implemented in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71749.