Empty From-Adress when sending mails out of ISPConfig without a running mailserver

Hello folks,
I just setup ISPConfig on Ubuntu 18.04 (ISP-Version 3.1.13) using a mixture of the "Perfect Server"-Tutorial and the Multi-Server-Setup written in the manual (used nginX instead of apache).
The manual suggested to have the "Master"-server to be only a webserver and nothing more (which I did) using only a barebone postfix installation.
By the time I've installed ISPConfig, I didn't know which Relay-Mail I would use, so I left it empty.
Now I've tested a bit and setup a mailbox using Mailcow (for now ) and have ISP the credentials for that (even through this https://www.howtoforge.com/postfix_relaying_through_another_mailserver tutorial). Well. The mails arrive, but have an empty Return-Path and an empty "From"-Header.
Is there a way to change that withing ISP which I just haven't found yet or do I have to reconfig postfix on the server directly?

Greetz