StefanRi25 If your user count is 5 or less, you could use the community version of Xeams. This acts as a full SMTP server and commercial grade SPAM Filter. Can run in Linux, Windows or Mac environments. I originally had it sat in front of an Exchange 2013/2016 server, and when I migrated to Grummunio all I had to do was change the Xeams configuration to tell it to send incoming emails to my new Grommunio server instead and my email were flowing in again and still fully spam protected, within a couple of minutes without having to recreate rules. Actually, I have never had to create any rules in Xeams, the defaults are really that good (for my needs at least). Admittedly, I have had to occasionally block a few email addresses and add a few keywords that have managed to get through the default filtering, but that is just a handful in the last 10 years, making it really easy to use with a low admin overhead. Daily reports of possible SPAM and blocked SPAM are easily configured for each users (auto emailed to them upto 7 times a day) so they can handle their own spam lists where they can ignore or recover if a rare false positive is detected.
Xeams also has a very easy DKIM setup, you just enable it on each domain the Xeams server hosts, and it generates the required DNS string you need to add to your external DNS servers. Also supports DMARC and MTS and all regular spam prevention options (SPF, etc). It has full Let's Encrypt certificate support as well within the GUI (albeit HTTP mode or manual DNS at the current time).
I have nothing to do with Xeams or the company behind it (Synametrics) but I have been using the Community Edition of Xeams for more than 10 years now and have always found that it compared very favourably to many of the commercial products I came across in my IT career. It is well supported and free to use for upto 5 users - great for most home deployments. If you need more users then you would have to pay, but that is similar to Grommunio. I run it as a Proxmox VM, using a headless Linux build so it has a very low footprint. Also it can keep a searchable archive of all incoming email - depending on settings. I have it dump all spam and possible spam emails after 30 days but keep all good email, which I then archive yearly - you know just in case!!) as it keeps the server storage requirement lower.
Using Xeams makes the Grommunio Appliance really easy to deploy as you do not need to deal with RSPAM (and its rules) or make any changes to Postfix other than the default settings during the Grommunio Appliance setup. The Xeams GUI (via web browser) allows really good control of your incoming emails and anti spam requirements. You can even use it to send the incoming email to more than one Grommunio server (I actually do this myself) and it is all handled through its GUI.
If you have not come across it before, give it a look. Yes, it will add to your deployment time, as you have to look at a new product and configure it for your needs, but once done, your spam admin overhead will drop massively. Apart from setting up my Proxmox Cluster one of the best applications I have installed - not forgetting Grommunio of course.
Hope above provides some food for thought
