So you set up the SMTP relay via Confconsole? If so, does your SMTP relay give any further info? If they're showing up in your relay as blocked, then I'd assume that it's your relay that is blocking them?! You may need to explicitly allow them to send to you or depending on your SMTP relay, perhaps you need to send them from a specific address?

TBH, I'm no Postfix expert and I just leave mine be (and they come from "root@hostname" as you note but I'm fine with that as they're only system messages and I know where they come from because of the hostname). Third party apps such as Bookstack should have their own config to set the from address (they'll use Postfix, but only as an MTA - mail transfer agent).

If you want to set a default from address other than root@hostname though, that's possible. I'm not familiar with the config OTTOMH though, so I've had a quick google for you. To my eye, this Unix StackExchange post looks like it should get you over the line? Failing that there are some other answers to that same question which may give alternate paths?

Then there's also the Postfix docs. Then because TurnKey is based on Debian, there is also the Debian wiki page (warning: often the Debian wikis pages can be a bit dated - but are still often worth a look). And whilst Ubuntu isn't binary compatible with Debian, it is still based on Debian (they import the source packages from Debian and build them on their own tool chain). So generally Ubuntu docs can be valid and useful too (just don't install anything from Ubuntu PPAs on TurnKey - unless it explicitly says that it's also for Debian).

I hope that at least heads you in the right direction. Please post back if you continue to struggle and/or share what works if you can work it out from those resources. If you get it working and post back, then perhaps we should create a new doc page and maybe even look at updating the Confconsole SMTP config plugin to allow setting it up a bit more?