User unknown in virtual mailbox table

Hello all,

I need some help with my fresh installed YunoHost-Server.

My YunoHost server

Hardware: VPS bought online
YunoHost version: 11.2.14 (stable)
I have access to my server : Through SSH and through the webadmin
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : NO
If yes, please explain: NO

Description of my issue

I have a freshly installed YNH server. The first two users have been created. The admin and a first test user. When I send a test mail to these addresses from a completely different mail account, I get an error message:

Recipient address rejected: User unknown in virtual
    mailbox table (in reply to RCPT TO command)

The diagnosis does not return any error messages. There is also nothing unusual under Tools > Logs. I have also restarted the server. And all packages are up to date. The switches for the e-mail services are also activated under the domain.

What could be the reason for this? Where else can I look for errors?
Does anyone have any tips?