Case
Our client cannot create cases from the incoming emails.
Solution
The reason of the issue is that the email was written to one mailbox, but there was a logic on the email server side that all emails were transferred to another mailbox and only after that they were synced with bpm'online via the first mailbox. As a result the second mailbox was a recipient of the email and the incident wasn't created.
We've added the alias to the correspondent mailbox in the "List of mailbox for case registration" and everything started to work as expected.