Forwarding email account means that all emails sent to e.g. sales@your-domain.com or support@your-domain.com are forwarded to the same built-in email address support@mail.your-account.ladesk.com.
But how does LiveAgent know in which department the ticket should be created ?
Let's say sales@... email address should go to Sales department, and support@... should go to Support department.
LiveAgent looks at the headers of email. First it checks To, Cc and Bcc headers if it contains one of the forwarding email accounts set up in LiveAgent.
Then it tries to find email account by alias attribute of the LiveAgent email account in email headers Delivered-to or To, Cc or Bcc.
As the last option it tries email addresses that it find in Received headers.
If there is still no match, i.e. no email address from the email source matches an active forwarding type email account in LiveAgent, the email is ignored and discarded. This is usually spam anyway.
Often the built-in email address support@mail.your-account.ladesk.com is found in the last step in Received header, and therefore the email is created in the department corresponding to the built-in email account.