SPAM seemingly originating from userid

Hi, I am getting spam which appears to originate from a user on my server. Can someone explain if I understand this right? See mail header extract:

Received: by ws.bario.com (Postfix, from userid 30)
id 8745C2537C; Thu, 3 Sep 2020 01:48:12 +0000 (UTC)

I am interested in the "userid 30" - does this refer to a plesk userid (e.g. email account)?