SOLVED:
I wasn't using my Cloudron instance as mail server. I have some ISP issues with inbound email so I try to avoid cloudron mailbox for now. The good news is that for the SoGO login doesn't need a fully working mailbox just an existing one. The domain mentioned in the manual is the dns domain(!!)
Anyways the steps which fixed the issue:
For this example let's assume your domain is hostedservice.page and we will create a mailbox james@hostedservice.page
- Go to Cloudron Dashboard
- Under profile select "Email"
- Edit the domain of your choice which in our case is hostedservice.page, where you want to host the mailbox. You'll find it under Domains → Domain → Actions → Edit
- Under Email configuration hostedservice.page you'll find Mailboxes
- Click on Add and select a name like "james" and assign a cloudron "owner" as the Cloudron dashboard user account who will own the mailbox.
- Click on the created mailbox and you will see something like james@hostedservice.page
- Go back to SoGO and use the mailbox username james@hostedservice.page and the cloudron user password
- You are in. That's all.
Thanks for the help!