Web Proxy usage for SMG services - no AV pattern updates


I'm trying to get an SMG running behind a web proxy system. The connection works because I could update the OS via the Web Proxy.
Now I send the first mails through the system and notice that no patterns, AV and SPAM, are loaded by the SMG. I see this in the logs of the gwava and in the Web GUI under "Manage Server".

Has anyone ever had the case before? Or tips for further analysis?

Thank you & Greetings